How can I stop being "too nice" and get better at speaking up for myself / being confident? I struggle with being myself since I've been hurt in the past.

I've been told I'm "too nice" and want to shake the label off. I'm a kind person, but I don't want to be perceived as a bad thing. I am bad at telling my romantic partners/friends when something bothers me and I end up getting resentful. I know I need to get better at this and am not sure how to improve.