How to say NO

I have trouble saying NO. I meet a guy I've been speaking to and I mentally talk to myself and say I'm not going to sleep with him or give him a blow job or anything and I always don't leave without doing it. I just feel pressured like I have to I know deep down I don't have to but I've always been such a softy in my life. I'm not saying the guys force me or anything but I just feel the pressure and I really need help on how to stand up for myself better .. and itsnot about I can't stop myself cos I'm horny it's more about the guy being disappointed (not like I care) but idk it's really weird so I've stopped seeing guys until I can work on this