How to let go of anger, resentment, sadness towards my boyfriend?

hey everyone, my boyfriend met two girls in October last year and never told me about it. he started DMing one of them memes and never told me about it. i asked him who she was when i noticed he was following her and he told me he has no idea who it was. weeks after i asked i went on her instagram and there was a picture of them together, him and the two girls. i asked him about it and he said sorry and it was just a white lie and he’s sorry he didn’t tell it and ain’t do it again. i understand DMing the opposite gender and sending them memes is not a big deal but it’s the fact that he lied to me about it. also i also feel like im not a good girlfriend to him because he always tells me that im annoying by asking so many questions about the relationship and making him feel less than worthy. so in a way i am insecure that he met another girl. i also feel like he wanted to talk to them to get close with them and be with the girl he was DMing. I don't know, im always in a bad mood around him now but not with my family which is weird. i was to forgive and forget. I don't know where to start or even how to do it
How to let go of anger, resentment, sadness towards my boyfriend?
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