I need help

So I’m kinda new to the relationship world , always been about me myself and I sooo I go in a relationship thinking about a “ we “ and “us” every time we get into an argument it’s my fault because I’m sensitive I guess . & this last argument was about a girl he added on his snap . I asked him who she was and as he was saying she was from high school his voice got squeaky . I creeped and did some fbi searching and stuff . Come to find out no she didn’t go to the same high school . She’s two years younger ANND we graduated almost 5 years ago, so why now add her ?? . I’m contemplating my next move . What would y’all have done ??