Married and thinking about ex

My husband and I have been married for a few months over a year now and the past few weeks I haven't been able to get my ex out of my head. I've had thoughts flooding through my mind about what if I married my ex instead of my husband and what our life would be like. I don't know what is causing all these thoughts and feelings. I keep praying that the Lord would take these thoughts away, keep my faithful in my marriage, and change my attitude but I can't help these thoughts. 

Has anyone else experienced this?