We all have to take responsibility and ownership for our own actions and decisions in life. You have your own life to lead, and that means that you can't watch this girl 100% of the time. I can see that you feel you have a duty, or a responsibility as such to watch over her, but you can't in reality do that all the time. Because life is demanding, and you can't live two lives at once. All you can do is make sure she knows that if she ever needs to talk to someone, you're there. If she needs a friend, you're there. If she relapses, as you say, then you're there to turn to. And hopefully, she will come to you if she needs that shoulder to turn to.
If you think of it, so many things could happen in life, and you can't prepare for everything. That's the thing about life, it's unpredictable, it's random, it's unknown. All you can do is live your life to the best of your ability and know, in your heart, you're doing your best, for you and your friend.
When a friend of mine was depressed and talking about suicide, that's exactly what I did. I set up the network, so she knew that if she needed me, I was there. And I got on with my own life. She told me that when I was fussing too much, and worried too much, it actually made her feel worse. So, I backed away a little, even though it felt that it was the wrong thing to do, because it was actually helping her not having me there too much.
Your friend is lucky to have someone like you around, who cares.
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The only good thing about breaking up
Is the making up we do after