Boyfriend problems I need help :(

So basically like two days ago I found out my boyfriend has been talking to this girl for over a month behind my back and kept it from me cause he didn’t want me to get mad he made a girl friend. The reason he thought I’d be mad is because once I was uncomfortable with him talking to this girl I didn’t trust and the way she talked to him was too forward and just weird. I haven’t done anything else though anyway he reckons they’ve never had a proper convo but talk everyday and night until like 3 am and go to the same uni and everything. The whole situation has destroyed me cause we never kept anything from eachother and this was a big secret for quite a while, everyone I know including myself agree she seems to be into him but he’s totally in love with me I just don’t know what to do I feel like I’ve been cheated on but I don’t want to make him feel too bad cause he was just stupid and didn’t realise how wrong he was. Please help and give your best advice I’m really sad and lost