It's up to you if you want to write a journal.
I wrote suicide notes, both a generic "Why I did it" one and more specific ones addressed to friends and family.
On the one hand, it delayed my CTB because I'm a perfectionist and I am spending so much time editing them.
On the other hand, the fact that I have messages prepared is one thing off my CTB checklist so I feel more prepared to do it.
I recommend writing your suicide note(s). You might get the same effect and it might help you commit to it if you decide you want to, instead of delaying it with "well I can't CTB yet if I haven't written a note."