Why do I think about my ex?

I’ve been with my husband for 2+ years and we have a 1 year old together. Before him I was with my high school sweetheart for 4 years, but he cheated 2x and it was toxic. I still find myself thinking about him and how in love I felt. I love my husband and I feel awful that it’s been this long and I still find myself thinking about my ex. But, when I first got with my husband I felt excited and at ease. I’m not sure if it’s because I got into a relationship without fully healing first but why do I still think about him like this? I wouldn’t leave my husband, I just don’t understand why I still feel this way and why it doesn’t feel sufficient enough with my husband. It just makes me feel so bad. Will this ever fade?