When an ex makes non-stop fun of your new boyfriend?

Well, if you go through my past questions (don't recommend it, it's long, drawn-out, and boring), me and my ex who have been super close at one point are struggling at the "friends" stage. Basically, I've been throwing myself at him while he was in a new relationship (big mistake) and as soon as I decided to pull away, he insisted on being friends. He knew how I felt about him, and I got the feeling he wanted me as a "back burner" situation, aka, if things went awry with his current girlfriend, he'd gladly start back up with me.

Anyway, I saw the situation for what it was and nipped it in the butt. I went out and did my own thing instead of waiting around for him and met a really cool guy. He is VERY successful in his industry (owns a recording studio, worked with some major artists) and this somehow makes my ex very insecure. My ex already knew my feelings for him and he shot me down. But now that he knows I'm dating a fun, successful guy, he's behaving very differently. He will not stop talking crap. He's never even met my new Boyfriend yet but he will still talk endlessly about how he is "emotionally stunted", "terminal adolescent", "douchebag", etc.

How am I supposed to respond? We're in "friends" mode now, but he had his chance with me. He turned that down and now that I'm trying to move on he is being mean as hell and I just need to know the appropriate way to respond.
When an ex makes non-stop fun of your new boyfriend?
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