Is my boyfriend cheating on me or am I just overthinking?

I’m young in a quite serious relationship and we love eachother very much but shit has happened. At the beginning of our relationship, I got an Instagram dm from a. Random girl wondering if I’m dating the guy iam (let’s call him josh). She was saying she’s joshs ex and that they still call every night and sleep on the phone and he knows she is very much still in love with her etc. i confronted him about it and he blocked her well at least I thought. I was being clingy for a while but I noticed everytime I’m clingy a guy doesn’t want it so I stopped being clingy and now all of a sudden he’s texting me at 3am telling me he loves me? That’s guilt. Anyways so I went through his phone yesterday and found out he’s texting her again and I really can’t do it anymore. How do I break up with him
Is my boyfriend cheating on me or am I just overthinking?
Post Opinion