He was "with" another girl at the same time

im really upset because I recently found out that my new boyfriend was with another girl while he was with me but before we were officially together (we were like friends with benefits? idk). The most they did was peck. I know the girl and she is very flirtatious. He said they'd hang out and then sometimes they'd just end up cuddling but he never hung out with her planning to do anything (so he says, but idk). Apparently sometimes she'd give him a peck and several time he did the same. I remember at one point he got distant from me, apparently that was cause he really liked her. Then he realized what a bad person and friend she was.

I know we weren't together and they didn't necessarily hook up. He has apologized but I'm hurt...i don't know what to think/do
He was "with" another girl at the same time
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