A guy I was seeing still lives with ex who messaged me telling me what he said about me. How can I stop blaming myself?

Number one, other people's opinion of you are none of your business! That guy messaging you, why would he say that? He's got some motive he's trying to fulfill, and it isn't anything good for you. You can examine yourself, learn the hows and whys of the relationship not working, and plan for future relationships, and grow as a person. But I don't think you should blame yourself for some gossip. Focus on you, and be the best friend you can be to yourself. When you're in a relationship, keep working on that, you'll learn more and more all your life. What others say about us, in the end, never matters.