I need advice

So my boyfriend and I have been together for a few months. Everything has been perfect and amazing. He makes me feel like I'm on cloud 9. But this morning after he left for work I got a message on instagram from some girl. She screenshotted a message he sent her about a week ago and it said "my light saver is ready for you to use the force on it". The time stamp on the message was when he was sleeping over and I was sleeping at the time. The girl said that she didn't reply and the second she noticed he had a girlfriend she tried finding me on Instagram to let me know that he did that. I'm grateful that this random girl told me. She said that she can't stand guys like that and wanted me to know what a slime ball he is. I have never suspected him of cheating. I've never snooped through his phone out of distrust. I love him so much. I don't know what to do. He said that he has no idea what he was thinking and that he will never make this mistake ever again. Idk if I should forgive him. The last relationship I was in was all a lie. The guy cheated on me so many times. I truly thought that this guy was the one. I still kinda think he is. The girl said that he must have found her on tindr but he swore he deleted tindr when we started dating. What gets me is that I took his virginity, he says he loves me more than anything, he says he never wants to have sex with anyone but me and he even talks about marriage. Please give me some advice on what I should do. I'm losing my cool over this.