Relationship Advice

So my SO and I were together for almost two years when I was fifteen to when I was seventeen. A year and a half ago, I was hospitalized for suicidal ideation and ended staying in an residential treatment center until December of 2017. My boyfriend and I never really ended it or anything before I left, we still loved each other very much but obviously I needed help and my leave was sudden and he was not made aware of where I went after I was hospitalized. He had no idea if I was even alive. I left my residential when I turned 18 about five months ago, and I called him as soon as I got out and could. He told me he still loved me and never really stopped and I realized that I felt the same towards him. So we started dating again after not talking for about a year (not by choice obviously, I had no connection with the outside world while I was in my rtc). He makes me extremely happy and I the same to him, but there’s one thing that’s kind of holding me back. When I was gone, he hooked up on again off again with this girl. When she told him that she had feelings for him and wanted to be in a relationship with him, he stopped everything physical with her. He said it was because he still loved me I guess. They still talk though as “friends”. While I was gone though he kept pictures of me up in his room, he kept posts of our one year and six month anniversary etc on his Instagram... so she knew that he had been dating someone. I don’t really know if I should be okay with them taking still, I’m not really a controlling person, but it really bothers me. I’m not mad at him because I know it was such a difficult year for both of us. He has told me that he got pretty depressed after everything happened as well, but we’re both so happy now. This is just the one thing that keeps messing with my head. Like she’ll keep commenting borderline flirtatious comments on his posts, and when I tried following her on social media, she asked my bf who I was and when he told her that I was his gf, she denied my request. I don’t know if it really is weird behavior or if I’m just being sensitive or what. Any tips on how I/me and my SO can work past this so that we can be the strongest couple we can be? (I’m 18 and he turns 20 in October by the way so we’re not super old but not exactly high school aged sweethearts anymore ig)