Oh GS, you are in such a difficult place right now, I'm so sorry and so behind on reading threads that I'm only just catching up on your situation and goodness, I have been in these dark places myself and they are tough, tough times.
Re the wedding, I am wondering whether just an honest phone call, or perhaps email if that is easier, explaining, as you do so well, that your health is not great, plus the situation with your son is making attending difficult. I struggle with letting down (as I see it) people who have been good/kind to me but also know if people really get you they understand where you are coming from. And then perhaps a lovely gift with a nice letter reiterating how much you appreciate your cousin's kindness, now and in the past, and perhaps some sort of tentative plan for you to visit them when you are feeling better and your son can go as well (or whatever situation works practically)? You could have a sort of wedding re-run going through the photos and video of the day at some later stage.
I really resonated with your post about resentment leading to self sabotage; I do think the difficulties in my life have been my way of saying "Look what you did to me, look how much you hurt me" to my parents - not consciously of course but it makes sense to me that I couldn't say they caused me all that pain and be having a wonderful time. I'm very painfully reminded of my mum dismissing my step-father sexually abusing me, her argument basically being that as I'd had a child I'd obviously had sex so the abuse couldn't have been too bad. I'm starting to wonder if that's why I've been single for so long!
With regards to those terrible black moods, which I used to suffer from frequently, I used to find that focusing on the next ten minutes got me through. In the next ten minutes I'll clean my teeth. In the next ten minutes I'll put some clothes on. In the next ten minutes I'll eat a piece of toast. Just one chunk at a time, one little, achievable goal in that time. I remember how bad it felt so my heart aches for you.
(((((((((((((((((((((((((((((((GS)))))))))))))))))))) sending you strength and lots of love xx