What do I do now? One day he loves me the next day he hates me?

Anonymous
Basicly, me and my ex broke up beacuse apparently he was always making me upset and he wanted to make me happy, we ignored each other for a while and then for the weekend we started talking and ALL weekend we were always talking ans acting like best friends saying "love you" and then yesterday he went out with this girl who he liked and now he's treating me like he doesn't know me? from "I love you xxxxxxxxxxxxxx" to "bye x" how do I regain our friendship?
What do I do now? One day he loves me the next day he hates me?
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