I have been in a relationship for two years and have a child with my partner, why is it I can't stop thinking about my ex from years ago, I miss everything about him, I love my boyfriend but I'm constantly thinking about my ex please help and when we were together it wasn't the greatest of relationships

You said yourself that it wasn't the greatest of relationships, so perhaps you've built your ex up as a person who he isn't and probably never was. Your current relationship could be lacking in some form or quiet frankly you're just not happy with your boyfriend, which inturn sets your mind back to other times and another man!

My advice to you would be to dig deep and find out why you're truly feeling like this because no one here can give you the 100% right answer.