How do you learn to say no to sex

I have gotten a good amount of my symptoms under control after a lot of hard work. I’ve gotten much better at people pleasing in general, but the area I still struggle the most with is sex. It’s like whenever someone touches me in a suggestive way I enter a state of dissociation so severe I can’t communicate that I don’t want it. I keep ending up in situations where I know if I said no he’d stop but he doesn’t take the time to ask if it’s ok in the first place. Usually this would be fine and all but i am petrified to communicate that I want to stop every time. I feel like hes not going to stop and knowing that is scarier than just accepting it. Or he will stop and be mad at me because in my brain that’s the only thing he wants me for.
For context my partner isn’t abusive and cares about consent a lot and I know wouldn’t actually be mad but I just feel trapped behind glass