I don't have the same attachment to a house, but living on a different continent than my entire family, far from where I started my adult life, I do have my heartstrings stretched at times.
So here's a tiny tip that works for me when I need to manage the emotional attachment to something: I keep a symbol of it, that is, I keep the emotion. It could be photos, a physical piece of the object, an invite to an event, something like that. For instance, for your house, I may simply take some really great photos to display later, with or without the family you raised in the picture. Actual good photos you could frame and hang in the new house. It'll serve just as well as the symbol of your badassity throughout the years (and could be a conversation starter!) Another way would be to salvage a distinctive part of the old house and transplant it in the new house, either as a useful implement or as art/decoration. You'll keep the emotion, but you'll get rid of the mortgage. You're not really "getting over it" this way, but why would you? It's an important part of you :)
Wow, I'm feeling all hippy now. With this kind of tip, YMMV even more than usual, but I hope it helps!