Personally, I struggle with this often.
I was in a relationship for four years with a person who was emotionally manipulative, psychologically and physically abusive, and overall, took me away from much of my life, family, and friends. Somehow, though, I still find myself analyzing our past relationship, finding a fault of my own, and second-guessing my decision to cut him out of my life for the better. It's easy to get caught up in feelings of "what-if", and second-guess your initial judgment to call it quits - time and distance not only makes it difficult to remember a person, but also feelings and scenarios. Your negative reactions slowly push further back in to your mind, and eventually, only good memories spark up when you imagine your ex.
The best thing to do is to take a moment to re-evaluate your ex, your relationship, and yourself.
What was the relationship like? Honestly.
How did you feel during the relationship?
In comparison, what kind of person are you version the kind of person you were?
In comparison, if you are in a new relationship, how do you feel versus how you felt?
Did you and your ex have any tangible future together? Did it work, truthfully?
Imagine your best time together. Now, imagine the very worst time. How do they balance? Was it worth the ratio?
I always forget how toxic he was until I really think about all of our worst times alongside the good times that plague me. I force my thoughts to those of his abuses, rather than our spiritual connectivity or other good qualities of the relationship. I also realize how much better of I person I have become since exiting that very distressing connection, and finding my now-boyfriend. And finally, I compare the man who loves me now and how he loves me to the way that my ex treated me on the daily.
If you find yourself seeing them solely in positive light? Maybe it is time to reconnect.