My child's dad cheated on me for another girl

My child's dad cheated on me then left me for this girl he's always wanted for years. He is treating her like a princess and they seem so in love. It's only been 3 months. I no theyll be together for years that's what hurts the most. I feel so stupid and she's posting constantly about him and pictures of them looking so in love. He's spent thousands of pounds on her already. He never see's his daughter maybe once every 2 weeks if she's lucky. How can he do me so wrong and not care at all ? Is it true once a cheat always a cheat. He cheated on me through my pregnancy too with someone else :(