I think that sometimes I accept that what other people say takes a priority over my own time or my personal choices. Also I'm not so "fighter" type. For example, suppose that if I go to rent an apartment, and it has some imperfections. I prefer to accept them than ask or direct the owner to fix them. I end up taking my own time to fix them. Thirdly, if a person asks me something and I am already doing something else, I will try to help them. Even if they overlap I will say "yes" to the other thing or perhaps completely drop my own project. So that also takes time.
So, my question is how you say "no" and fight back (see that I'm talking about my rights, not an actual fight with bare hands... and perhaps other suggested types of fight back "for get my place").