under what circumstances would you break up with your partner even though that you still love them?

I was with a lady for 5 yrs. We were completly in lust with each other, but probably never in love. We eventually broke up, but arguably should have done that after a year when I discovered her cheating on me, and so started cheating on her as well. (childish I know ) On hindsight we were never right for each other. That said the sex was great and there were a few wonderful memories of when the times were good. The prob was that the relationship was never terrible, so we just continued in our flawed but acceptable way. Except for the sex and the good times, probably a waste of 5 yrs.