I think I'm too nice and my friends take advantage of me.
What do you do for your friends?
I let them borrow my money for stuff they never return
While it's your thing to be nice to people and it's not a bad idea either, those who make it their habit by getting money from you that way are just plain stealing. Get rid of these false friends and take them to court for borrowing and never paying back before you end up in the poorhouse.
Okay maybe.
I understand you may not want to leave them since they're friends, but sometimes I believe you have to realize they are only using you and that you can be too nice. Why not try standing up for yourself once, it will feel amazing to have that much control and power over your relationships!