Can you record some stream of consciousness thing to help collect your thoughts when writing?
Exactly what I was thinking, one evening when I was in a bad place I just opened up a word document. Not intending to write a suicide note but I just wrote whatever came to mind. Thoughts, feelings, why I was feeling so bad that night. No thinking about particular bits, really just throwing everything I was thinking and feeling onto paper. Pretty cathartic if you ask me. I was listening to some depressing music too at the time. I think that's an optional step but it might help you "tap into" that part of you. Obviously skip this step if you feel like it's be dangerous for you to do so.
Went back a few weeks later to realise most of it would make for a good note. With some editing I think it came together alright.
Another thing I've done for the past year or two is write a note and read it back every few months. Editing and rewriting as needed at a time I had the mental energy to do so. Certainly not something you need to get right the first time.
It's definitely tiring and not something to rush, it could even be just a couple of lines to start off with.