My ex is in a relationship with the girl he told me not to worry about?

Literally the title. My ex is now dating the girl who, at the time we were dating, he used to always comment about and express interest in. I always felt cautious about his feelings towards her. And I ended up ending things with him, not because of her but because he messed me around.

He has attempted to get me back but I know things haven't changed. Even up until a month or so ago he was trying to contact me and liking my pictures on social media etc.

Today I've just found out they're together. I feel devastated. I feel like he never really cared for me and was waiting for her or something. He treated me so carelessly. I feel like an idiot. How can I stop feeling like this
My ex is in a relationship with the girl he told me not to worry about?
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