What should I do

Tess • I am really short and I am in a band and I'm 18 and I love everyone
I found out my boyfriend Has been sneaking behind my back and meeting up with girls, he and my friend say it's only to vent but idk. Why would you have to sneak around to talk to someone? Why would you not want to talk to me? I am just confused and at this point don't know what to do and I'm just sad and in denial. He knows I've been cheated on and swears he hasn't been and all that stuff but I'm just stuck. I hate breaking up with people but others are saying I should be more understanding. If anyone has any advice that would be really appreciated:)