I’m curious, other than the ex-girlfriend telling you he is cheating, is there any other evidence that it’s true? What makes her such a reliable source of information?

But I feel for you in noticing that you might be drifting apart. People change. Relationships ebb and flow, rise and fall. If someone other than you is the center of your universe, the risk increases that you may sucked into a black hole after the intensity of the light dwindles and the star goes supernova.

One advice that i have learnt the hard way “never make anyone your world for once they are gone you are left with nothing”.

Yes the girl could have motives, but then you have started to notice change. Would he let you look at his phone? see who is messaging?

Because you love him it is very hard for you to leave. So there is no point to look for evidence as you have stated you will forgive him and believe me once he sees how easy he has it he will do it again. You either face it and do something about it or let it go. Truly you are not going to have a peace of mind for a while.

Just love yourself and be strong.

I went through the same thing a few years ago. I didn’t listen to what his ex gf had to say. Because I was so in “LOVE”.
Before you knew it, I was his ex gf and I was the one calling his new girl to tell her the same thing I had been warned.
Forget him, move on. You’ll be better off without.