He’s playing with us 😔

So I’ve been with this guy for 7 months now, since the beginning I made it pretty clear to him that if we were together we had to be exclusive and he agreed. But for the last 2 months he’s been different with me. He doesn’t ask to see me like before, he goes missing for hours, and is just not the same. Well I know he had been talking to his ex fiancé since August, but he said it was only cause her father had had a stroke and he wanted to be there. Long story short, I know he’s still seeing her even tho he denies it and even after I told him that if he still had feelings for her I’d back down. I have strong feelings for him, I’m in love with him which is why it’s hard for me to back down and just leave him. I honestly don’t know why he won’t just admit he is trying to reconnect with her and let me go, like just end things already. One of the strongest reasons I’ve also stuck around is because I wanted to get solid evidence of what he’s doing. I just don’t know if I should tell the other girl that she is also getting played. I mean if I were her, I would want to know. I’m not doing it to be mean or hurt her in any way, but she doesn’t deserve it. Especially if he had already broken her heart once before. Guys idk what to do. I’m in complete shambles.