If you've sold your collection

[IreneL]

Maybe it could be a new start for the collection, putting a more limited and clear focus to it?
Decide that "I am only collecting....and...." and let the rest go?

If I get a pony that is not really in good enough condition for the collection, a double, or something not included in the few focus areas, I don´t usually have a problem to sell. Even if I am at first tempted to keep them for the collection. I just remind myself. "I am not collecting these. Someone else could have better use for them!" Which of course is a lot easier with the right motivation and if you have truly decided what to focus on.
There is a ton of other ponies out there that I find adorable, so it is a constant mini-struggle against the urges of having just because they are pretty, or special in some way. But it could be more cool to create a narrow collection with a theme than just have it all.