this is normal but not necessarily healthy. for me i spent all day repressing my thoughts with tv, work, music, etc. and once it's bedtime and i didn't have those, my thoughts came roaring back because my brain didn't have anything else to focus on. the things you can't stop thinking about usually stem from an insecurity or a past event that hurt you. if this applies to you i'd recommend reliving some memories and letting yourself feel them and understand them. every time you process an emotion, it gets weaker and hopefully the thoughts will decrease