I need some advice

So I don’t have anyone I can really ask this question of and hopefully people aren’t going to judge on this. I have a lot of trauma from sexual abuse I suffered as a kid and as a result I struggle with men and males when a romantic or sexual relationship develops. I have a guy from work who I fooled around with a couple times and I decided to stop it because I was getting uncomfortable but he has started it up again and I don’t…….I don’t know how to tell him no…..I didn’t ever really learn how to do that kinda thing because it never really…..worked and now I’m terrified and crying because I’m so scared of what to say or of him pushing me to do something again. So my question is……how do I tell him no?