If saying 'no' is an issue for you, start asking things back: small favors here and there. Just to let people (and yourself) know that you have needs too.

The hidden thoughts behind the behaviors could be:
- I must always say yes (or they won't love me, could be the potential threat behind that)
- I must aways do for others and never ask: i'm the strong one here!!

I like helping people too. But there were times when i NEEDED to help people out. That's different. I had a feeling of being of use, after helping others. Without the helping, I didn't feel like existing, or that i had a right to it.

If you have people taking advantage of you is because you don't respect yourself and you can't teach others, just with your behavior, how to respect yourself.
It's quite a common issue don't get discouraged.

I think you should focus on respect. What is respect for yourself and for others. It is important that you respect others but it's basic that they respect you.

If those boundaries are too difficult to settle, a counselor could be a good choice