I think part of the reason it's so hard is because it's giving up on the final "could have beens."
Even though those "could have been's" are more likely "will never be's" it's still difficult.
You're not crazy.
You're not alone.
You're not the first one going through feelings like this.
All I can say it keep reminding yourself of all the good in your life now, and of the reasons you left in the first place.
You will feel better.