Who was wrong in our relationship? :(?

Well, I think that was wrong of him to start dating someone KNOWING you guys were willing to work it out eventually. Him doing that is not a good sign.But "getting back" at someone by texting someone they hate is never a way to solve anything. You guys are acting impulsive and if you want a real relationship, you're gonna have to develop some self-discipline, or you will all do some damage to yourselves and each other. You need to tell him he hurt you when he started dating. Relationships are hard, but if you BOTH are willing to push through and work it out, it will be a beautiful thing. You also need to tell him WHY you started talking to that man, then stop communication with him. But if you ex is not willing to work to restart the relationship with you, there is nothing you can do to save the relationship.