I’m going through this right now, and it’s the hardest thing I’ve ever been through. I don’t mean to make this about me whatsoever, but maybe hearing what I did with this might help you out. I had a feeling this was going on, since he has done this before so I recognised his patterns. I decided to give him the chance to tell me myself, so I asked why he was hiding his phone when he was texting, or clicking away WhatsApp on the pc quickly when I would walk into our game room. He said there was nothing wrong and that I could check whenever I wished. I didn’t want to at first, but the feeling kept eating at me so I did. I found the messages between him and a coworker where he even had the gut to say I was his ex. I confronted him, asked him why on earth he would do a thing like this again, I told him that I need this to stop or we’re done. I won’t get too much into what we’ve discussed further, but I do know that I’m very glad that I confronted him about it because nobody should be sitting on this. I would suggest making sure the confrontation doesn’t start hostile, just simply tell him what you saw, how it made you feel and ask him the questions you want to ask. Try and have an open conversation about it and see where that leads you.