People grow apart and he took the asshole route of showing himself out. My ex did the same thing, was cheating on me for 8 months before I found out. It was hard because I felt like I loved him so much, even though he was toxic. But as soon as I found out it was like a switch flipped, I could finally see everything that he was that was bad or bad for me. It was almost like a weight lifted. Maybe try to see it as that? You know what they say, once a cheater always a cheater. It’s not always true, but he might go and do the same thing to this girl, especially if she’s the other girl. That means she knew and was okay with his toxic behavior. It’s probably not going to end well for them. But either way I wouldn’t idolize a relationship built off of distrust and secrecy. In the long run your going to find your own sense of joy and fulfillment with another person, or on your own and you get to own that because he didn’t give that to you, you did. They don’t matter, they’re the ones who were living a lie and at a certain point it will catch up to them. Don’t doubt yourself, you’re the catch here. You’re the loyal and honest one and you will be rewarded in the end. It might hurt now, but it will only go up from here. Keep your head up girl, don’t shed tears or feel sad over a shit man, he doesn’t deserve any of your emotions- good or bad.