How do you stop being too kind and become more confident/dominant?

I've noticed that I've spent most of my life being the "nice guy." I avoid conflict, struggle to say no, and often stay quiet even when someone disrespects me. It's not that I'm weak physically, but in social situations I sometimes freeze, overthink, or don't know how to react.

When someone is rude, I usually let it slide. When I want something, I hesitate to ask for it. I worry too much about what people think of me and end up putting other people's comfort above my own.

I don't want to become mean. But I do want to become more dominant, fearless, confident, and respected. I want to be able to speak my mind, set boundaries, stand up for myself, and stop acting like a people-pleaser.

For anyone who used to be like this, what actually helped you change?