laelithia,  You said …

I feel horrible that when I am with my current partner, I often imagine how much enjoyable whatever activity we are doing would have been if I was with my ex instead. I don’t know how to rid myself of these horrible thoughts that haunt me. Do you have any suggestions on how to do this?

….

…in my current relationship, I believe it is a shadow of what I could have had with my ex. Every day I think about how my current life could have been so much better with him in it, with us together. 

It seems like you are doing your current partner a disservice by staying with him. You are not being fully present with him.  You are not really IN the relationship with him, rather you are really still “with” your ex.

My suggestion is to heal yourself around your ex first before being in another relationship.

Mark