Book recommendations in that respect:

- "No more Mr. Nice Guy" Robert A. Glover: A great book on identification and elimination of nice guy behaviors with lots of exercises.

- "When I say no I feel guilty" Manuel J. Smith: Techniques with lots of examples on how to stay assertive and deal with attempts to manipulate you into doing something. Relevant for dealing with colleagues, spouses, shop clerks, ...

There are others but those two books will get you very far if you take this seriously.