First, many thanks to everyone who commented. The outside perspectives have been of great help, and have given me a clearer idea of what's going on both in my current situation and also inside my head. I asked my father to make me an appointment with my GP to see about some help, and linked him to the post by way of explanation. He happily agreed, and expressed pleased surprise at everyone's responses to my post: they were all so thoughtful and measured, so thanks from him too.
I saw my GP this afternoon, and she's referred me for a preliminary appointment to work out what sort of help (counselling, psychological or psychiatric) will be best. I've also been prescribed antidepressants, which I'm happy about despite having heard horror stories about them. It's going to be a long and slow process, but at least I've started on it now.
As for the events that prompted all this - the saga of Rob and Bob - I still don't know what I'm going to do, but based on my own instincts and everyone else's suggestions, I think for the time being I'm going to try avoiding them while I try to sort my own head out.
So, thank you all again for helping, and for making this community such an excellent place for people to get the help and perspective they need.