Do they move on to better?

[Jabbas]

My ex left me for somebody else and I keep thinking she is better than me/prettier than me etc and it is affecting my self esteem. Is this a normal feeling?

Well, she is happy to establish a relationship with a committed man. So I suppose it depends what your definition of better is.
You're wounded and it is natural to want answers and we often turn into ourselves to seek the answers.
But the reality is that some people don't have the staying power for a lasting relationship and would part company at some point, nothing to do with the physical beauty or lack of their partner.
Some people just grow apart and it's about the dwindling of the connection not the attributes of the partner, and our connection or lack of is a two way street, no one can maintain it alone.

Try not to torture yourself with what she has that you don't. Remind yourself or get some friends to get you remember that his lack of faithfulness is not a good measure of your worth.
Can you look in the mirror and know you would like to be in a relationship with you? Are you a good friend and a kind person? That's what counts in who we are.

Thank you

To be honest she isn’t much to look at but maybe they just have a better connection

Maybe. Connections can be based on lots of things. The psychology is interesting.
Sometimes we feel 'connected' to people because they don't expose our flaws and we can relax even if that means we continue to behave in ways we ourselves don't even admire.
Sometimes we feel 'connected' to someone because we can assert out wishes over theirs and they submit to our preferences at their own expense.
Sometimes we admire something about them and are flattered they are paying attention to us, while they may be using the relationship as a sticking plaster for their own problems.
Sometimes it's one lovely amazing person vibing with another lovely amazing person.
Sometimes it's someone who fits our ambition of status or self image and we acquire them as part of the package we seek, happily willing to overlook less savoury parts of their character...

So so so many reasons why two people can feel connected... Not all are the stuff of romance novels and not all go the distance.

Try not to dwell on it (easier said than done), can you distract yourself and put your attention on other things until the intensity of the pain starts to fade?