Awful. I am so sorry for your loss, OP.
A friend of mine killed himself about a year ago. I know I can't relate exactly, but I hope this little bit of perspective can help:
Your mom was in a bad place emotionally, and she was obviously very sad, so don't go clouding your thoughts with the idea that you could've done something to prevent this, and that you're somehow responsible. I know it wasn't helpful for me when I was grieving, and is the wrong way to look at it.
Again, my condolences to you and your family.