Your ex is your ex for a reason. He has chosen to start dating a new girl.
Whatever your feelings, it is very disrespectful and unfair to his girlfriend if you're pestering him now, when it's clearly over. You could cause problems in their relationship, when he's none of your business anymore.
I know it's hard to move on, but once someone has a new girlfriend it's really not cool to message them, and the fact he's not currently replying to your messages tells me that he doesn't feel the same way about you.
Sorry, but the sooner you move on from this, the better.