Thanks @Sanbeiji for your post.
It makes a lot of sense and you are right, I can't help them if it breaks me in the process. I have been in the position that you describe before - and yes it got worse, and yes to a point of life and death for them - the level of involvement I had for a while involved giving up my own life, and yes I prevented their death at the time, but when I began living my own life when they appeared to be in a better place ... it happened, and they died and I felt guilty. But I gave up my own life (ambitions at work, seeing friends and family, hobbies, etc.) for them at that time because I loved them so much and I got a lot from the relationship we had, it wasn't one sided, it wasn't actually draining but I didn't have freedom to live my own life for fear of what would happen.
I would like to extract myself from this 'friendship', and so far I have been firm enough to limit the amount of time, 10 ish texts in the morning, 1hour phone call once a week and a face to face meet up has been sporadic but not frequent due to covid and I haven't done anything to change this since things have opened up more. I am there at other times when ad hoc things come up to upset them and they call - I try and make the time then.
I falsely hoped it was fizzle but she does put the time and effort in to keep it alive because she needs it.
I think my guilt around it stems from my own happiness, I have a lovely family, wonderful easy happy kids, copious friends, am happy at work, am happy at home, and my only real complaint in life is that I have too much to do to fit into the day which can be stressful but in the scheme of things I recognise just how lucky I am. With this in mind my mental health is good and I feel I should try and help because whilst I find those interactions draining, I definitely have the capacity to take it on. I only have time at the end of the day on the whole so I can go to sleep after and reset for the next day so it doesn't derail a day.
I think I have a plan now (thanks to a suggestion on this thread) to continue the interaction but redirect the interactions into something jointly positive.