I looked up the chapter that deals with the issue in the biography of Willem II by Prof.Van Zanten.
The 'wedding project' started in 1812, before the Orangist turnaround in the Low Counties. The Orange family and their advisors saw the wedding as a way to get the family back on the royal stage; they had been without a country for 2 decades.
The pro-Orangist revolts in 1813-1814 meant that the family's luck had changed & they were invited back to The Netherlands. Advisors of Prince Willem VI (later King Willem I) claimed that the match had become 'unwise and untimely' almost exactly from the moment that the prince regent had spoken warmly in favor about it in february 1813.
Willem II was informed about the wedding project in June 1812 when he was on a military campaign in Spain. He did not pay much attention to it as he thought it was impossible for his father to succesfully arrange such a match. In august 1813 he expressed his concern about the polluted atmosphere within the British Royal family to the Duke of Wellington. In november 1813 he arrived in Plymouth, under the impression that he would quickly travel on to Prussia. He could not hide his shock when he was toldd by minister Bathurst that the prince regent and his father had made new arrangements for the alliance. A day later the prince-regent forced a first meeting between Charlotte and Willem at a dinner party in Carlton House.
The prince described the princess as 'a beauty a la Rubens' but wrote that she had: 'an unpleasant voice, stuttered, made strange movements with her head when she was talking, she walked heavily and noisily and her ways are not very refined'. He did like her cheerfulness during dinner.
After dinner the prince was asked by the prince regent if he liked the princess and if he wanted to marry her. Willem of course answered 'yes' but added that he would like a meeting the next day to ask the princess if she had any objections against the match. The prince-regent said that such a visit was not needed as he claimed the princess had just told him that she liked the prince and that she was happy about the choice. He ordered the princess in the room and asked her in front of Willem: isn't it that you agree to the marriage?' Willem noted that she started crying, the prince regent took it as a confirmation and joined the two hands of Willem and Charlotte and had them hug each other. After that he left the room and a stunned Willem and Charlotte did not know what to say to each other. The prince asked if she agreed to the wedding out of free will, to which Charlotte said that she did. She added that she liked the little she had seen of the prince so far and that she was charmed by the prospect of their marriage.
They two met for a second time and talked about common friends. But the third meeting a day later at Warwick house was a disaster: the prince told Charlotte that after the wedding they would have to divide their attention between their two countries, which meant that the princess was required to stay in The Hague a few months per year. The princess started crying and ran away to her bedroom. Apparently the prince-regent had asked Willem not to tell her about this arrangement until after the wedding and the princess appreciated his honesty (as she wrote in a letter to Mercer Eliphinstone). A few days later Willem left for The Hague.
He went back to London on April 29th 1814. There was a cordial meeting with Charlotte, who 'lied like Cassandra' & assured him she wanted to go ahead with the wedding. The Duke of Sussex had started to oppose the wedding due to the complications now they both were heirs to a throne. They were supported by the Whigs, who had some scores to settle with the regent. The next days the princess refused to receive the prince, even though he called on her for several days. On May 3rd he wrote to his father that he was : 'I am very confused and so nervous that I can only write to you with great difficulty'. The behavior of the princess made him feel like he had landed in a bad comedy where he played the role of an undesired suitor.
On May 9th he received two letters from Charlotte. Lord Liverpool tried to reach a compromise with the princess in which it was stated that her father and future husband could not require her to be in the Netherlands for more than two months per year. the princess refused.
On May 17th Willem wrote a letter to his father. Above the letter he wrote 'very secret, but maybe show to mama'. He asked his father to pressure the prince regent to indulge all requests of the princess. He gathered that the end of the engagement would be very disadvantagious for Willem I who was busy necociating with the great powers to add the Southern Netherlands to the kingdom.
For some reason it seemed that the princess changed her mind once more. By the end of May it appeared she had given up her objections. She allowed him to visit her again, and on May 24th he wrote to his father that he was seeing the princess every day. Lady Bury -a lady-in-waiting- was surprised by the changed attitude of the princess. She thought that the reason was that Willem stood up for Charlotte in front of her father. But she added that Charlotte's affections changed with every way the wind blowed. The two went to various balls together and appeared arm in arm.
On June 6th the prince regent had received a letter of Willem I where it was suggested to drop the clausule from the wedding contract about her stay in The Netherlands. The regent was not happy with the proposal at all, but agreed as he was afraid a break down of the engagement would lead to a scandal, especially as many crowned heads of Europe had gathered in London to celebrate the downfall of Napoleon.
When her father agreed, Charlotte changed her mind yet again: she realised that a wedding meant that her father would 'win'. On June 16th she told Willem she could not leave London as her mother needed her. A disappointment to the prince, who hoped that she would come to the Netherlands for a short while after the wedding. After they got into a fight about a carriage ride his patience was finally over: he walked away without saying a word while Charlotte was holding a sermon in which she demanded him to join her in the carriage. In the evening he received a letter from Charlotte who came to the conclusion that the engagement had to be cancelled. A relieved Willem sent a copy of Charlotte's letter to his father and wrote that the princess had behaved shamelessly & that he was glad he got to know her true character before they got married. He added: 'I could never have lived together with her and it truely is a blessing from heaven that the case took this turn'.
He replied to the princess two days later. He said he informed his family about her breaking of the engagement but he refused to inform the prince regent, as she had asked. King Friedrich Wilhelm III of Prussia -who was in London & an uncle of the prince- suggested he would leave England so he would be less exposed to gossip. But the prince stayed around for other festivities.
The strongest rumour was that the engagement had been cancelled due to the coniving of Grand Duchess Catherine Pavlovna of Russia because she wanted to marry Willem herself. The rumor was not true (she was to be engaged to a different Wilhelm -of Wurttemberg) but both the grand duchess and her brother the Tsar had taken a liking to Willem. They often visited him and tried to save the engagement of Willem and Charlotte. The Tsar himself spoke to Charlotte and tried to convince her to receive Willem again.
The Dutch envoy wrote that the prince regent had underestimated the opposition to the match but in the end blamed ' the evil character' of Charlotte, 'a virago with free manners'. The earl of Mamelsbury wrote: 'it makes you wonder about the greater evil to which such a character and such an attitude of a person with such a high position can expose this country and its inhabitants in the future'.
Upon arrival in The Hague on June 28, the prince seemed depressed. The king did not give him much sympathy and reproached him for 'lack of ambition'. For Willem I a wedding of Willem and Charlotte would be the crown on his work. For 20 years he had tried to restore the fortunes of his family. He even expected his son to return to London to give it another try. He added that a man could never be unhappy in a marriage because 'he could always find distraction elsewhere'. The prince did not obey his father.
Instead he travelled to Brussels. On July 5th he promoted Willem to a general in the British army and offered him the command of the British and Hanoverian troops in Belgium. George thought that Charlotte would turn around, but Willem refused to return to London. A year later he fought at Quatre Bras and Waterloo. His father tried to set him up with one of the daughters of the Austrian emperor (Leopoldine or Clementine). It would have been a nice gesture to the Southern Netherlands, but in the north such a catholic match did not find great enthusiasm. A Prussian princess was out of the question as the relationship with the Prussian king was at a low due to different interests at the congres of Vienna. But the biggest obstacle was the prince: he now refused every suggestion that his father made, in a way channeling his former fiancee, Princess Charlotte.
In the end the prince himself arranged the match with Grand Duchess Anna Pavlovna of Russia, sister of Tsar Alexander. He liked the Tsar and other members of the family and was perhaps dazzled by the splendor of the Russian imperial family. The King was not enthusiastic about the match but he found it impossible to refuse the suggestion after he received a letter from the Tsar. Anna later told baron Mackay that for her it was 'love at first sight'. For royal standards the marriage turned out to be relatively happy, despite Willem's many 'distractions'.