[Dashir]

You are all wrong on this one, I think.
This is a classic case of "the-good-old-days". It's a myth. You over romantisize the old relationship, spend your time reminiscing over the times you spent together and create this grandiose image of who they were. This results from your being out of touch with them, going through some tough times and holiday blues.

The cure is to talk, at length, with the former partner and you will see them as human and current as opposed to angelic and perfect. Stick with the one you are with.