Hey Ant,
When you say you're struggling with letting some of your games go, are you struggling with actually selling them or the idea that you just won't have them anymore? In other words, do you like having them for the sake of having them and being able to say "check out what I have" or do you geniunely like to play pinball?
If you genuinely like to play pinball, you'll probably miss them. The good news is that if you do miss having them you can always buy a few games again. If you like the idea of having games but don't really play, I don't think you're going to miss them. In other words, if they're just possessions, you won't miss them.
My advice would be to sell a couple and see how you feel. If you don't miss them then keep on selling them. If you really miss them then either enjoy what remains or buy a few new ones to keep your interest up.
Dave