Hi again packed lunches I totally understand when you say the POTs threads do not quite fit your circumstances. Apologies, I should have explained that I was recommending them for the advice about how some parents cope in difficult , stressful circumstances rather than for the issues suffered by the teens themselves. Eg self care, getting more support, stepping back a little (not possible in your circs), ADs etc, pursuing some hobbies and interests out of the house just for yourself, and most of all, sharing and being able to talk openly about your worries with another parent in similar circumstance.
I thought that maybe you would identify with the feeling of helplessness and emotional "burden" (I know we don't see our DC as burdens but they can sometimes provoke a crushing feeling of responsibility, frustration and almost despair) that some of those parents feel. But I understand that you are also seeking solutions for your ds too.
I have had a look around and there is a series of threads aimed at parents of anxious teens which may be more appropriate for your situation? This is the fourth one here and so there must be previous ones somewhere:
www.mumsnet.com/Talk/feeling_depressed/4187552-Parents-of-anxious-kids-teen-support-thread-part-4
I hope they help and fwiw, I know it doesn't help to say so, but I think what you are experiencing is a very natural response to testing circumstances. We do only feel as happy as our unhappiest child! That saying has a lot of truth in it.
I think we are reluctant to discuss these issues in RL because our DC are older, we are wary of betraying their confidentiality and there is not as much support out there as there is for parents of toddlers who are struggling for example. But if you can talk to a professional or even another parent, you might feel better?
I am not an HCP and have no experience in this area but I would be wary of a diagnosis of purely situational depression if you have been experiencing these episodes since childhood. That sounds more like some sort of disorder like ASD or ADD or similar? I shouldn't speculate as I have no idea really but it might be worth seeking advice and maybe saving up to pay for a proper diagnosis privately if that is at all possible?