am i dumb ? Need encouragement

my ex boyfriend of 3 years left me 3months ago. He decided to reappear New Years night where i demanded he give me a apology for the words shared between our departure in which he did. Days later it’s his birthday and he asks me for sex.... i figure out he’s with the girl who i thought he was messing w prior to him cutting me off. So why isn’t he fucking her & deciding to try to use me. I just need some encouragement tbh cause I’m still feeling some type of way from the breakup and the betrayal .... & must i add there’s a new man in my life (& i don’t want to continue a pattern of cheating on him with my ex)