Ended up dating the guy she told me "not to worry about"

I'm 25, me and my girl called it off 4 months ago, our families weren't compatible, both were very orthodox, hence we'd be wasting each others time if we stayed together any longer

We broke up mutually and still stayed in touch via dms, met once and everything seemed normal for 3 months

She stops texting me for a week, so I asked her if everything is all good, she responds telling she's dating her guy friend and is already in a relationship

She used his name after our breakup to make up scenarios and make me feel like shit, then turned around and is actually in a relationship with him, I know its not supposed to be affecting me but I feel like shit, idk how to cope