Missing someone I shouldn’t miss

I’ve been in my relationship for about 5 months and recently it’s really hit me hard when I think about my last relationship before this. The two relationships are totally different from each other and I just don’t know how I feel anymore. I love my man so much, we’ve even talked about marriage in the future. I just don’t know why I can’t get my ex out of my head lately. It’s really starting to affect me on a mental level. I find myself comparing the relationships and... I don’t know. I don’t know if there’s something wrong with me or if I’m being stupid.