I (26M) found out today that my girlfriend (26F) of 6 years has been meeting my best friend (28M) for sex over the past 6 months. We have just bought a house and collect the keys this week, but now I don't know what to do.

Sorry if this post drones on a bit, I just feel like I need to clear my head and write everything down as it will help me to deal with what happened today.
As mentioned in the title, my girlfriend (whom I will call H) and I have been together for around 6 years, and thus far have had the perfect relationship. No big fights, great holidays together, the lot. In November we put an offer in on a house which was accepted, and we are due to pick up the keys on Friday this coming week.
However, this morning I found messages on H's phone between her and my best friend of 11 years (D). They were sexting, and making plans to meet up for sex. D is in a long term relationship himself, and recently purchased a house of his own and a couple of months ago had proposed to his now fiancee (S). Obviously this shook me, and I confronted H, who confessed to me that it had started in August last year and they had met up 6 or 7 times to have sex in his/her car, usually late at night when myself and S were not around.
She broke down crying and admitted that it had started with flirty texting, before becoming much more serious. He initially was the one that started the flirting, but she enjoyed the attention and the excitement and so didnt put a stop to it. This lead to them meeting and her giving him head, and then over the next few meetings having full sex in their cars. However, after their last meet in December she had got cold feet and told him she didn't want to do it anymore. She also told me that this was the only person it had happened with.
I then left her house as I did not want to be around her anymore and wanted some space. I am not a violent person at all, and have never actually been in any form of physical fight, but I drove to D's house to confront him as i was shaking with anger. He was not in, although S answered the door and could see I had been crying and was clearly very angry. About an hour or so later, I receive a text from D saying that S told him I had been at the door and was clearly upset, and it could only be about one thing. He said that he had told her everything, and that we needed to talk.
In the meantime, I had been out for a drive to try and call myself down as it relaxes me, and had picked up another close friend to confide in. He convinced me that violence is not the answer, as it's not worth getting in trouble with the police / risking losing my job over. After driving and talking for a few hours, I dropped him home and drove over to D's again with more of a plan in my head.
I knocked on the door and he came out and broke down crying. We drove around and I made him tell me everything, start to finish. I drove to the house I'm supposed to be buying, and told him I'm not sure I want to buy it anymore. I told him he's wrecked that home before I've even received the keys. I called him a snake, and told him that he had broken my heart. I took him home and told him not to contact me again.
Finally I messaged S and asked her how much she had been told. D had said to her that he and H had met for sex one time in August, and not since. I then told her the full story. D and H had both told me individually their versions of what happened, and both stories added up so I believe this to be the truth. Obviously he was not man enough to tell her everything.
Right now I don't know what to do. I don't know who to trust, and I don't know what to do about this house. We have already exchanged contracts so we technically own it already, but I still don't know if I want to move in to it with H when I can't trust her. I don't want to have to break up as I don't want to lose what we've been building together for the past 6 years, but I don't know how we can get over this.
I am utterly heartbroken, and I don't know who to talk to. I'm only writing here as I need to vent, although any advice about where I go from here would be welcome. Are there any professionals I can talk to about this?
tldr girlfriend cheated with my best friend. Not sure how to handle the situation.


Source.

So...D is giving her the, well....D