How to stop other people from using me?!?!

Hey everyone, I'm new to this site, friends told me here you can talk with others freely about everything plus you might get rewarded for it. This might not be a story, it'll be just a question rather. I wanna see your opinions.

I often ask myself why am I so good to everyone ? No matter who asks me for favor or anything i tend to just help them and do it for them. This all is good at start but when people see that I just wanna help everyone some of them just keep exploiting that and sadly I cannot stop helping them cus I am raised like that and I kinda feel the need to help other people. I'm not stupid of course and if I see that someone is using me for something dangerous or illegal i wouldn't do it ofc. I'm mostly talking about friends / relationships. No matter how good you are they always wants more more more and at the end they find some reason what it is not enough anymore to be friend or in relationship...

I wonder what do you guys think about it ? Did you have same experience ?