How Do I Keep My Ex As a Friend?

I met my ex a few months ago. This other taller leaner yet stupid retard guy met her a little while later. i quickly mvoed in and became her boyfriend. The other dude sat on the sidelines for a month or two. He constantly hit on my ex and my ex did nothnig to prevent it and protect our relationship. Eventually she pushed me away with excuses blaming me to be with the other guy. The other guy is garbage. I was living with her and her dad. This new guy she dating her dad kicks out of the house cause hes a total loser. She has now gone missing for a few days cause she ran off with him somewhere. Before this though she completely killed our relationship and treated me like ****. We fought a little much and she said **** the friendship too. She really didn't want me knowing she was dating the guy she promised wouldnt screw up our relationship.