What do I do?

Here's the problem, I'm a really nice guy but at the same time I'm kinda cold hearted. So the problem then raises, I have three girls that are dead in love with me and me with each of them but the problem above that is the person I love the most is with a jerk druggie and there is nothing i can do about it. The three girls are kind and everything but I don't want to live with the guilt of hurting them and living with that forever...(I know the guilt will not fade) and I dare not hurt all of them so I have know idea what to do. I may be good at solvings others problems but when it comes to mine....I'm no good at it.....To top the fact i have no selfesteam