I just wanted to let you know I read your message and that all sounds really hard. I know you say the docs are telling you your depression is situational and they can't help until that changes, but of course, if you're in a situation that you can't change easily, that doesn't mean there's nothing that can be done.
You still need to be able to cope with the situation and it's not just going to go away. So I supposed the reason I'm replying is just to ask, how are you? Have you thought of their ways you can look after yourself?
Treating feelings of depression doesn't always mean looking for drugs to help or he advice of doctors. Sometimes it's just doing an activity you enjoy, going for a coffee with a friend or making your favourite dinner. That probably all sounds obvious so I'm sorry if that's at all condescending, it's not meant that way. And of course it doesn't deal with the situation with your son. I really hope the advice given above about mind and samaritans helps. I know Sane also has a helpline (open on the evenings). Have you thought about going to a councillor for yourself? Or perhaps family therapy? It may help your son to realise the impact this is having on you? Remember you matter too, and you need to have boundaries so you can find the strength to carry on.
Also, know you're not alone. There's lots of us going through difficult, draining and emotional situations with our families, partners, friends on here and writing in these forums really helps (I know it helps me and I'm at breaking point right now- honestly I feel like life is just passing me by and there's nothing I can do to help).