Hi yellow knight, welcome to PC.

Sorry to hear you're struggling. I don't have an obvious reason for my depression either, and that caused me to feel a lot of guilt that I was ungrateful and to feel I am a weak person.
Depression doesn't respect our situations or whether we're successful. Just think of Ruby Wax who is open about her depression.

It is a lot of pressure to support someone with suicidal thoughts!! I have a friend who talks to me about suicidal thoughts and has phoned me after attempting it. I want to support her, but sometimes it hurts me too. When I feel unsettled by it , even if not excessively bad, I phone my local support line (in uk it's the Samaritans). I figure that if I'm doing their job with my friend, they won't mind giving me support to do it, and they don't.
They are always happy to let me talk through my feelings and it does help. At one time I planned to phone them once a week (and did)when I was feeling the pressure a lot.

Beyond that I remind myself that I can only do what's reasonable (note not what's theoretically possible, but what is reasonable) and what they do is their responsibility, NOT mine.

If I had a friend who was physically dangerously ill and might die, I'd worry a bit and be concerned, but I couldn't change it if the illness took them I would know I had tried to support them. It's the same with mental health, if the illness takes them (by suicide), I can't control if they live, I have to just do what is reasonable to support them and look after myself and that has to be enough.

It may sound harsh, but if I felt responsible I would be in the same boat as them and be no help to them.

I wish you all the best. What I say is just my thoughts. I hope you get as much support and encouragement here as I have had.