Fake account

So me (18 yr old F) and my boyfriend (19 yr old M) have been together for 3 years this past October. Just yesterday I got on his snapchat and I seen he was talking to a girl.. I keep looking and it’s a fake account he made exactly a week ago(made sure of it , hasn’t been longer) . He wasn’t saying nothing bad and he was talking about how he loved me to this girl and the rest was just like friendly talk from what I can see. I also seen he added two of his ex girlfriends but they didn’t have no conversation at all he also added another girl and all he said was “single?” And she didn’t not reply. He was sleeping while I did this so I woke him and I felt so much hurt in my heart and started bawling. I left him and came to my moms and he keeps apologizing and he knows he fucked up and he said he didn’t do nothing physical or talked to them in any flirty way but this is still NOT okay to me. He has been crying and begging me to just listen to him and he said he doesn’t even know why he did that, that he just did and had no intentions on hurting me with this by doing this. I love him so much but when I’m in a relationship I want to feel secure by knowing how we stand in our relationship and the fact that I have lack of trust with him doesn’t help with that at all. I know I’m worthy of more and im enough and a good person to be loved right but I also want to know I’m the only one they’re interested in, only girl he has his eyes on, ( I don’t want to be with someone’s who has other girls on his mind) which he says he only wants me but I don’t see where the necessity was for him to have gone to make a fake account to do that and he said he doesn’t know why he even did it, that he just did it and that he knows he fucked up but he wasn’t doing anything “wrong” just friendly talk ( but I can only see chats not actually snaps). I want to be with him but a huge part of me doesn’t want to deal with that but I feel like we still got so much to do together. I’ve also felt like I have gotten too dependent on him and I don’t feel like I know how to be alone no more which I want to work on because I really do dislike a lot about myself and I want to love myself and be okay with myself but it’s just like he is my best friend and it’s just so hard to think about us not being together. I need advice on what I should do basically stay and work it out with him OR leave and work on myself and see how that goes. TL/DR “Boyfriend of 3 years made fake account exactly one week ago to talk to a girl. Nothing flirtatious was said and he mentioned me and talked about him loving me. Still did it behind my back. Lack of trust for him and I love him and he loves me but I’m not sure still being in the relationship is good for me.”