Was something wrong with me?

He cheated on me with a girl he met online, flew out across the country to see her.. They've only been together for a couple of months and he already gave her a promise ring & they're living together.. I can't seem to understand why he gets to be happy with no responsibilites yet I'm left here in feeling so sad having to bust my butt off because he couldn't be man enough to help me raise my daughter :/