I can't say "no" and it's ruining my life

I might be exaggerating about the last half of that, but I seriously have this inability to say no. When I was SA'd, I just froze. I laugh things off because I don't know how to react. It takes me a day to even figure out how I feel. Everybody else knows what I'm feeling long before me. When I try to set boundaries, they just get totally trampled down. It's like I don't even have a voice at all.

Maybe that's why I write about mute characters so much.

I'm seeing a therapist and we're working on stuff (finding a different place to live, dealing with dissociation, keeping safe whilst in crisis, etc). But like

My friend just asked me something. My BEST FRIEND. Of almost a decade. And I can't even say no to her.

How the heck am I supposed to learn to say no now?!

I'm so tired of being a total pushover. It makes me resentful and avoidant of social interactions.

Please, I really need some advice. How do I say no to people?