hypothetical: break up in exchange for someone you care about more?

It's called grass is greener on the other side syndrome, and the majority of the time, it's the same brown colour on both sides.

You think you love them more, but in reality you just haven't had to put up with their **** as long.

 

There's a massive difference between lust and love / intimacy and infatuation basically, though the two are easily confused.

 

Yeah, Joe loves Monkey Island, lusts for a new game and is infatuated with Guybush Threepwood

 

I'd recommend getting out of the current relationship, because if a person is cheating on someone, whether physically or emotionally, then that someone deserves better.

I'd also recommend that person come clean about their actions, so the other someone knows for certain that it's not their fault and they really are better off.