It's habit and you have been in the habit of backing down and/or letting others walk on you. It will take quite a bit of work to change that. I would ignore how you "feel" pretty much and keep letting other people know they are not to walk on you, tell people what you like/do not like, set boundaries and consequences that you like and that help you. You are just here to be "you" and get your life working the way you like it, not to please others.