Hi rough, your post really struck a chord with me. First things first, it's absolutely not true that you're of no interest to anyone. Your DS certainly won't think so, nor will your DP.
Having all the nice things in the world doesn't guarantee perfect mental health. I read a quote from Stephen Fry recently where he suggested that depression isn't necessarily a reaction to bad circumstances, but more like the weather in your brain - sometimes it just is. It can't be turned on and off magically any more than we can decide we want it to stop raining and expect that to happen. But you can put your umbrella up until the rain goes off. That's what people who feel rotten, like we do, need to do.
I go on and off with my mental health. Sometimes I'm fine enough, but other times it will just seem to kick off for no reason. I know for a fact, though, that one of my very worst depressive / anxious episodes ever was caused by the prospect of getting married last year. Nobody was making me stressed out, everything was well organised, but I was so afraid of letting people down, of ruining DP's day, or of breaking down on our honeymoon. I was so, so sick last year, and it was terrifying. In the end, nothing bad happened on our wedding or honeymoon. They were both brilliant. But don't discount the stress of just wanting to do your best and being afraid that you won't.
If the doctor's been useless before, is there another GP you can see at the practice? Or, if you write down the way you feel in bullet points, you could take it along to them and let them see how rotten you're feeling. I think a lot of us are so used to putting on a brave face that we find it hard to drop it sometimes, and we end up looking OK to the doctor.
Do PM me if you want to chat wedding stuff - whether lighthearted or serious. You're not alone.