Setting boundaries is something I've always struggled with. I think it's the little girl in me who wants to be liked, so she'll do anything or put up with anything.
Of course, that's been a recipe for disaster. I would eventually get resentful and then angry. By time time I said anything to address the issue, I would be so angry that I wasn't acting in a reasonable manner.
From the other person's point of view, my reaction usually surprised them. I was acting angry about something that I had said was ok (when it really wasn't), and then next thing they knew I was suddenly acting resentful. Of course they thought I was crazy!
Sound familiar? Yeah. It's taken me a long time to get to a place where I figured out that it's best for everyone if I speak up sooner, rather than later. It's ok to let other people know what makes you happy. The key is to speak your mind with kindness, without anger, so that people focus on your message and not how you're acting crazy.
By doing this you're establishing a pattern of honesty in conjunction with kindness and self-awareness. Plus, all those times you felt like a doormat and full of resentment will go away because you're taking ownership and responsibility for your own happiness.