Why did my ex choose the other girl over me?

I was with my ex for 3 years and we had our issues and occasional fights. But when I found he was talking to another girl, he never told me about it and I co signed on an apartment with him just for him to choose to be with the other girl and tell me that he still wants me in his life. I chose to burn the bridge because I was hurt but this new girl has spread lies about me and he seems to believe it

Why did my ex choose the other girl over me?
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