I don't understand the need people have to think ill of exes. Why is he "unworthy" or "insincere" just because he's moved on faster than you? Why is that? There's nothing wrong with letting the good times you had together just be good memories. Why suspect them, too? It's so unnecessary.
Look, I'm sorry you're having trouble moving on, but his moving on is just another assurance to you that moving on IS the right process. He's not hurting you, he's living his life. Stop thinking good OR ill of him. Just work on YOU and work on moving on.
And people, we need to be a little less slow call "foul" just because exes have moved on, when that's what we're counseling OPs to do as well. What's good for the goose is good for the goosette.