It's not really about being better than him (or her) though, is it? It's about being happy with your Self. Maybe you need to figure out who you are. What makes you as an individual?
You are looking towards him for validation.
We can pull apart your 'self' beliefs, but if we tell you why are worth something, you are still looking for validation from external sources.
Think about it this way, do you look at other women and think 'oh her husband left her because she's...
... pathetic, unattractive, stupid, desperate, idiotic, mumsy, fat, boring, nagging and generally a pretty shit human being'?
I am sure you would be kind to another woman in that situation so why not be kind to yourself? I do believe that it is worth figuring out how to love yourself. It takes a long time and lots of work, IME, especially after something as painful as this. It is doable and once you achieve it you will look back on this and wonder how you could have written those things about yourself.
The fact you have said those things about yourself seems to me to be part of your continued (understandable) desire to find an explanation for why he left. It's almost like saying, because I'm x, y, z then that explains why he was attracted to/picked her as she's not those things... Or that makes it logical/acceptable as she's not a nag, boring, just had a baby etc.
I don't think this is really about her. It's about him and possibly the DC. She was just a distraction who happened to be there when he was starting to waver about his life.
Another thing to think about is, even if you were all those negative things that you described, does that mean you deserved to be treated poorly? Being fat, mumsy, nagging, boring or whatever doesn't give the other person in a relationship a licence to treat their partner badly. If they do not like those things, they have many other choices they could make such as leaving one relationship without starting another, going to couples counselling, just talking about it etc.
You see your H as a different person to the man that you knew. Now you are seeing yourself as an individual rather than as someone defined by someone else/solely as part of a couple. You will probably have to work out and rebuild your view of yourself. Hang in there. You might also want to discuss this stuff with your counsellor as in sure they would be able to help you.