Have you been able to say no in a non confrontational manner yet?
I know getting overly upset is not always the answer, but you do have to consider your own feelings and be strong.
I have a smilar issue sometimes, but I don't feel like it's because I'm weak or needy. I'm just kind of "nice" and have been in a lot of other's shoes, so I'm understanding too.
If you've already been clear with a no. I hope it sinks in.
Please keep your own feelings in mind.