I was going through my boyfriend's old phone, and I can't stop obsessing over it
You obviously think this is serious.  Why are you obsessing over it?
My boyfriend has been talking to other girls behind my back.
It's wrong to invade your boyfriend's privacy by going through his private messages, even if he was talking to other girls.  It's wrong of him to talk to others, but he would also be justifiably upset with you for your snooping.  You should trust him, but ask him if he is still talking to other girls.
I think I found his profile on a dating app and now I am furious.
I think you should be concerned if you found his profile on a dating app. You really will have to talk to him.