I feel like a complete butthead

I was dating this girl recently for 4 months and our relationship ended pretty poorly, I blocked her on everything and left since she was toxic af. (not saying I was perfect). But I feel horrible because I’m not even sad over her, I’m legit only sad over the girl I dated before her and broke up with in October. My mom says it’s just because the newer girl was a rebound but I don’t want to believe that and just don’t understand how ending this one relationship can make me miss the other girl so much. I feel like an asshole for not being sad over the new girl at all.