Girlfriend Threatening To Hurt Herself if we break up?

I'll try to make this short. My girlfriend and I have been together for a little over a year (we are not teenagers, we are both over the age of 30). I am just not happy anymore. I tried to break up with her about 4 months ago, and she threatened to take her life, so I stayed. Now every time we fight, and she thinks we are going to break up she will threaten the same. I really want to end this relationship. I love her, I do, and she's a wonderful woman, but we just aren't made for each other. I know that when I try to sit her down and do this again, she is going to threaten to take her life once more. She has no family that I can talk to, no friends that can be near enough to help. I feel stuck. I don't want anything bad to happen to her, or for her to do anything drastic, but I can't stay anymore just because of the threats. I know that sounds selfish. What can I do, what's the best way to approach this?