My boyfriend has arranged to go to his ex's on our anniversary weekend and it's torn me apart

[Freya]

I have had a horrible life full of extreme family/other abuse and resulting trauma up until the age of 40. I've been married to a very emotionally abusive man, divorced and the man who came after committed so many acts of abuse and violence against me they ended up in the papers. Both my dad and brother (no contact for years) are violent sexual predators who despise females
I had a total breakdown in 2019 after said ex put me on the streets and was sectioned. Since this time I've been very slowly rebuilding my life and this is the first time I feel like I've had a life at all.
I have managed to find a small but very safe group of friends who have completely replaced my abusive family. I met another man, just under a year ago and the reason for this post, and fell in love. He also has serious mental health struggles, but he is not abusive. For the first time I managed to feel safe with a man. Recently, my life has become impossibly tough again.
I'm being kicked out of my home based probably on disability by an aggressive landlord, having to temporarily move in with a friend in a small space and part with my much-loved pet.
My best friend has had a breakdown and temporarily turned on me.
My job is putting unnecessary pressure on me and causing me a lot of stress.
The lawyers involved in the case against my most abusive ex have been in touch more than usual and (while I understand it's to help) the story is likely to make the news again which is obviously triggering beyond words.
My medication and/or the stress is making me really physically ill, doctor's investigations are pending.
My car, which I need to move house and rely on for my mental health has completely packed in.
Even the weather is hugely contributing, it's almost impossible to go outside due to snow and cold right now where I live and this is part of my issue I think of him going out to see her.
I have a long-term phobia of death, but this morning closed my eyes and wondered if I thought hard enough about it whether it would just happen

To move to the issue, my partner has an inappropriate close friendship with a blurred boundaries best friend/ex spanning a decade. We have split up once over this same issue just over a month ago and both hated it and decided to give it another go as we really do care very deeply for each other. As he explains it, it's more of a very close friendship where boundaries have been really blurred over the years and there have been 3 spaced out occasions of them sleeping together (for me the spaced out does make it kinda worse, but can't change past gone.)
To me, this feels more like an ex or even current partner/emotional connection than a best friendship.
Even her mother has said yes, they're in an emotional relationship right now (if we take the few physical encounters away.) He felt like he was in love with her for years but could never really have her, until finally somewhere down the line he accepted them as just friends.
This is his argument now and I do believe now that the friendship is now platonic. Some might say I'm naive, but I trust him as he's been so open with me about everything else and has proven himself mostly a very bad liar. She has said to me she feels like he is her little brother, and up until recently she was in a relationship with a female and apparently now identifies as gay. To add to the mix, I'm not even against polyamory and have tried it in the past, but we've both agreed this isn't what we're doing right now. I'm incredibly open minded as to all interpretations of what relationships mean, but this one keeps unfortunately coming back to bite me.

Recently, she also made me uncomfortable making demands of his time and using rude words in a message I saw completely by accident because he opened it while I was sitting next to him. Apparently they had a mini fall-out, in fact I know this because she appeared to have a slight go at him in front of me when we were all attending one of his friend's birthdays and then flounced off to another pub. All of this behaviour really upset me and he is aware. According to him, they sorted it out over Christmas and she actually ended up physically ill in hospital. She's also split with her girlfriend apparently on Christmas day, and as he has said doesn't feel like many people are there for her any more.

Only last weekend he and I had the most perfect weekend. I felt closer to him than I ever had to anyone in my life. We did speak about her, and there was one upsetting count of him absolutely having to get back to her message on the same day, but I think this was only one message in the morning and one in the evening. I still don't understand the same day thing, but it's not as if he were messaging her the whole day while we were together, far from it.

Overall, I felt like the air had been cleared and I felt secure about moving forwards. This weekend is our one year anniversary. She's got a busy job and is only free on weekend nights at the moment. I'm going to be really busy packing up the house and the anniversary isn't until Sunday so I had already said to him let's just see each other and celebrate Saturday/Sunday and I already knew she and him were making plans to meet up. I think even with the above to outright say "no you can't see your best friend who's just been in hospital" would make me controlling. It isn't the approach I want to take, I want them to evolve in their friendship as they are clearly really important to each other, just with more respect for me. So of course and as I almost suspected but hoped not due to weather warnings where we are and us all being told to stay home, he's going over to hers tonight (Friday.) I am in pieces and having constant nightmares. It feels so wrong that it's on the same weekend of our one year anniversary as he and I have been through so much, even breaking up over this issue but managing to rebuild. It hurts that it's at her house, a tiny part of me even wonders where it could go in that context one on one and if I'm being naive to ignore what other people might see are blatant 'signs.' But I've seen no sign of anything beyond a deep emotional connection myself so I don't believe it is more.

He has no experience of being in a long-term relationship, I am the experience other than a few flings and obviously he's been in this blurred boundary one for 10 years.

I am autistic and anyway also due to what I've described of my life above I have absolutely no idea what an appropriate relationship is. I don't feel appropriate asking him to just stay home and not see her when he hasn't seen her in over a month, she's been really ill (too) and when I'm not free myself, and when he's offered to see me and celebrate our anniversary Saturday/Sunday and help me move. But I don't know how I can move forwards with a happy weekend in the immediate aftermath of their meet-up tonight, how I'm going to get through tonight when he's gone for hours on end having his close emotional connection with another female. Mostly also when we're all being told to stay home due to treacherous weather. Clearly she is important enough to him for him to ignore that and risk safety, and that cuts much harder than a more ordinary "I'm just bored at home and we need a catch up anyway" situation. I just think the timing with it being our anniversary weekend is diabolical. I feel like I can't win. Part of me feels like maybe we will have to go our separate ways, but a bigger part wants to work it out if both people in the situation love each other. Is he behaving inappropriately? I know my feelings are out of control here and I know it's due to trauma, abuse and current life circumstances. I do also know though that they are still valid.

I wonder if even showing him this post will help, and I probably will, because I'm looking for genuine advice, not proof that I am right because I don't think anyone can be in this particular situation.

Thank you for reading this far and any comments :)

[ExpatInItaly]

There is no way I would continue a relationship with this man. 

I think because you have been terribly mistreated by the men in your life, your bar for acceptable behaviour is very low. No, this man isn't abusive but that doesn't mean he is good for you or that this is a healthy relationship. He is way too cozy with this ex, and I personally don't tolerate that sort of thing. Nothing has been worked out if he is still prioritizing her this much and you are still in such distress over it. 

I wouldn't tell him not to hang out with her anymore, but I would maintain my own boundaries and stop seeing him. This would not be the guy for me at all. I'm sorry.