flirty boyfriend

hey guys so i've recently found out my boyfriend likes to flirt. when we first started dating he was the sweetest guy i've ever met, so sweet but the thing that sucked is that he enlisted in the marines so he left for bootcamp when we wer 3 months into our relationship. a couple of months ago i was in his room on base trying to find his debit card that he had lost and i stumbled upon a notebook and there wer letters for me in there and one letter in particular caught my attention, and it was so beautiful and it said how much he missed me and all these nice things until i got to the end and i realized it wasn't for me, it was for another girl. and it basically said how he was so happy to have met her and how he wanted to spend time with her while he was on leave and of course it hurt my heart because this was the same girl he had kissed on christmas last year because she gave him a present but i didn't think anything of it because they wer friends and a kiss on the cheek is something sweet. but after reading that letter i confronted him and he said it was nothing and eventhough he made me cry and i honestly felt so worthless compared to her, he wrote in the letter how she could dance and how she was going to be a marine and how he wished he could be her, and i can't dance, im not a marine, im nothing that she is. last month my bf had for days off and all his marine friends wer planning a bonfire and i asked him who was going to be there and he mentioned all his friends but he didn't mention the girl he wrote the letter to but i asked him if it was okay if he didn't go mostly because of the drive from his house to there, it was a two hour drive and also it was in a sketchy part and he just got his new car and i didn't want anything to happen to it. and he ended getting mad at me for that, he didn't go and he was angry at me. i find out the next day that the girl was there and i confronted him about it but all he did was get mad at me, and i was the one who had to call him multiple times to get him to talk to me. so i was on his phone rn, and i see he had a message with the girl and he sent her a picture of herself through sc and it said "cute♥️" and i showed him and he just got up and left, he put on his shoes and said "ima go". so idk what to do honestly, im not mad at him, i just don't feel like talking to him, he's my first boyfriend so i've never been in this situation. idk if i should think of this as being a big deal or not..