Hi bazzinga, I'd say that you should really give yourself credit for trying to avoid the cutting. And trying to write was a really good idea, it can be very helpful for some people. But of course it might not be so much help for others or if the urge is just too intense.........
You could still write down how you've been/are feeling though, it may help to express it/"get it out there". And you could use that in confronting what's going on with you, whether that's for yourself or by passing on what you've written to a professional.
But for now, let's say writing was first on your list to stop the SI/reduce the urges. Sometimes it can help if you do yourself a list of five, six.......things you are going to do/try before you SI. So if writing hadn't worked/the urges were getting stronger then straight onto the next thing on the list e.g. a walk (or whatever you want).
But as for the wedding, well that's just one day. What's much more important is you think about yourself right now, give yourself some understanding if you can't make it. It really is your feelings/well-being that have to come first. If you're not going to make it, it won't be by choice, it won't mean you're to blame. So give yourself a break on that one, hey?
Maybe instead (if you can't make it) focus on doing something nice for her first anniversary. Doesn't have to be big, just "nice". I know it can be a long/hard trek to finding the right medication but just try to hang in there and do everything you can. I know it must be really hard but we're here for you if you want to talk more...........
Alison