I'd just like to start by saying your username is ridiculously appropriate for this sort of post.

On topic though, I used to think similarly to you in the sense that nice people are essentially taken advantage of by the general population. What I've noticed more and more lately though, is that there's a very significant difference between nice, generous individuals and what I like to call "welcome mats."

Be a nice person, but have limits. There's nothing that should stop you from being kind to others but refusing to accept others being unkind to you.