I agree, you can have healthier relationships. I'm working on it myself, and it's not easy, but I believe it can be done.
I had a recent incident with a friend and his ex-wife. They have a situation that doesn't involve me in any way. He wanted to spend the night with me so he could drive his ex-wife to take care of it the next day. She wanted me to drive her there so he wouldn't have to take her. In the past I was such a giver and felt I could save the world. I stepped back and realized I didn't cause this problem, so I should have no part in fixing it. I told him that clearly. I didn't tell her because she gets very angry and mouthy.
This is entirely different from a situation that doesn't inconvenience me, or a long-standing friend that really needs help. I'll gladly help where it's like a see-saw and they help back.
It is very scary and very empower to say No and stick to it. You can do it!