I am so sorry to read what you have been going through with your neighbour. It's a very difficult situation and you have let her know that you are there for her if she needs you. It is hard to stand by and watch someone trying to self destruct.

I am in the UK and the law here regarding suicide is different as it is no longer a crime. If a person has mental capacity, then they have the right to self determination, which includes ending their life. That doesn't mean you shouldn't try to help or dissuade them from taking what is a final step though.
It sounds as though you have been a good friend and neighbour to this woman who is clearly in such distress.
Take care of you too. I hope seeing the therapist helps you deal with the trauma of this situation.

Kind regards
E.