I believe relationships are about meeting not just the right person but at the right time. Sometimes it is not that your ex became a better person because of his new SO. It may just be that his new SO met him at the right time, when he had stopped acting a fool had really changed. It does not mean that you were a failure or failed to make him change. People change when they are good and ready and you cannot waste your time waiting for that. It may never happen. Sometimes people learn from mistakes they made with you. Second, sometimes two people just don't mesh. If you are not very invested in a relationship you may act out because you don't respect or cherish the match. Sometimes you know when you've met a good match because the person makes you want to be a better person and the feeling is mutual. If you were unable to bring this out in each other it may mean you just were not a good match. It does not mean you were a failure. Third, the person may not be better. Maybe his new girl is just more willing to put up with B.S. Than you are. If you deserve more and know it you will demand more and a man who does not want to give you more will be angry and feel nagged and unhappy. If you put up with his B.S. he will be a happy camper. So just because he moved on and is happy does not mean you would have been happy in that relationship. All in all it is good to do reflection as to your role in a failed relationship but don't use his new relationship to judge yourself. Only the people in a relationship know the real deal. How many times do you see seemingly happy marriages fall apart? You may be beating yourself up over what may not even be a representation of their actual relationship.