Cheating, verbal/physical abuse, alcohol/drug addiction, criminal behavior, constant arguing, long-term sexual & affection neglect.
However I've never been a fan of the "breakup to makeup" approach to relationships. If I decide to be "exclusive" with someone it means I'm going to give it my all. If for any reason it doesn't work out I have no regrets nor do I have anything new to offer!
If your best wasn't (enough) you were with the "wrong person".
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
Generally speaking people don't change unless (they) are unhappy.
There are only two ways to experience joy and peace of mind in relationships: We either get what we want or we learn to be happy with what we have. Accept them as (is) or move on.
Every ending is a new beginning!
Your future lies ahead of you and not behind you!