Thinking about my ex…

I got married at 22. It only lasted a year before we divorced. We were together for 4 years before we got married. I’m with someone new and after almost 2 and a half years of leaving that past relationship and NOT talking to my ex I still think about him. I have dreams about him. I’m racking my brain as to why. The man I’m with now is a dream man, I can most definitely see a future with him. I feel like I’m being a horrible girlfriend. I feel so confused, I know I’m still young, I keep thinking it’ll just go away but it hasn’t. Even after talking to my friends about it. None of them have been married so they have trouble relating. Any advice?