This is quite an important part, certainly for me.
When you're with someone you obviously create memories together based on places you've been and things you've done as a couple and these memories become important, cherished and also help bind the relationship as a shared experience.
The second the relationship ends, those memories are all you have left and because they've become so important and you want to cling onto them so much, they're thrown into sharp relief without the backdrop of the relationship itself.
Getting out there and creating some new memories is paramount in starting to build that buffer between the end of the relationship and the 'now' so the sooner you can do that, the sooner you will start to get on with the new you, sans fille!
And the worst part of it is that you only really appreciate and cherish said memories after it all ends. Scumbag brain working at it's best. It seems like a flaw with humans in general, as the cliché saying goes 'You never really know what you have until it's gone'.
Probably going off on a tangent here, but I don't think the current Facebook generation helps this matter, 'Oh look at what my partner has cooked for me... better take a photo of it and put it on Facebook' or 'Wow this is a great concert, better have my phone in front of my face recording the whole thing live so I can watch it later' and of course the popular 'I'm on a great holiday with my partner, better take an albums worth of typical portrait shots of us to post on Facebook so everyone knows we're having a good time.'
So rather than actually enjoying and being in the moment, people feel the need to try and immortalise feelings and memories, rather than letting them flow and follow their natural course.
So, learn from the past, appreciate the present and look forward to the future, but rather than me trying to sound like a preaching internet keyboard warrior, I'll let a monkey with a stick explain it better, true story;