Hi and welcome!
You are dealing with a lot, things that happened more recently and in the past and it makes sense that you would b sort of all over the place and have a hard time organizing your thoughts. It is a lot with which to deal.
You ask how to record things so you can start with a therapist (T). If you can write a few basic highlights, that might be helpful. You could even print out the first post you wrote here and take it with you as it paints a pretty good picture of your situation and where you are right now which is the best place to start with a T. A good therapist will be able to , over time, help you untangle things. They know that things are complex.
So I would suggest just starting with where you are at right now.
We have a family wedding coming up next week where he’ll be there. We’ve chosen not to stay at her house and just hoping there’s no drama for the sake of the couple!
Good choice on not staying at her house!

At the wedding, you can remember one of the easiest tools we suggest here which is
Don't JADE. JADE stands for justify, argue, defend, explain. Doing these things can often escalate conflict and gets you right into the drama which is precisely where you do not want to be. Read the article and see what you think.
We also have another article titled:
Escaping Conflict: The Karpman Drama Triangle which is an excellent read and something I think you and your fiancee would benefit by reading. Again, see what you think and then we can talk about it some more.