Search Results: boyfriend danced with other girls

Have you ever got into a situation that you think your boyfriend is the problem, but in reality, you're the problem?
That's me right now. Let's called my boyfriend A, and the girl he has feeling for B.
I've been with A for a year (we started dating in January) and in September, I started to notice he talked to this specific girl and I warned him that she had feeling for him but he didn't listen to me. I was uncomfortable about it so i started to give him attitude and we started to fight over petty stuff.
When we finally talked about our problem, he told me that he talked to his friend (B) more than talking to me because I gave him attitude, and he started to treat B as if she was his girlfriend (hanging out with her, going to concert, bring her to dance practice,...). When I confronted him about his feeling for B, he told me that he only liked her as a friend but when he posted her photo on his Instagram stories (twice), he always hid me and told me that he knew i would be jealous even though i told him i was cool about it. Plus, he didn't celebrate my birthday (or get me a gift) with me but he celebrated her birthday instead. He put a bunch of heart emoji in her contact, and they never stopped talking (Every single day). B has two Instagram account, one that she posted her personal but the other account, she posted a bunch of photos of her and A. She puts caption like "He's cute", "me and a routine: wake up, play game for 5 hours.", "get a man like him". It was really hurt for me to see those photos but why would he tell me that he didn't have feeling for her but she would post photos as if they're dating? Furthermore, he blamed that this relationship is doomed because of me, and he didn't change or anything. My friends said that i'm really feeble-minded to stay with him but I felt like it's all my fault in this relationship.
P/s: I'm 17 by the way, i accidentally put the wrong age on my profile.