I guess maybe I've done a good job of getting remorse out of my system from a collecting standpoint - possibly because my collection is still way too big?
Couple things that have worked for me:
1) I rarely ever buy something with the sole intention of resale, which I think has prevented me from making some really dumb decisions. If I do buy with resale as at least a potential motivation, it's usually still something that I'm interested in (player, team, set, something). If it doesn't fit into some portion of what I'm trying to collect, I usually just pass and that includes taking cards in trade. I do fewer deals because of this I'm sure, but it also keeps me out of deals I shouldn't be in.
2) I never think about what I sold once it's gone. If I sold it, I did it for a good reason at the time and got what I thought was fair in the sale. At that point, I'm on to the next pursuit - and experience has taught me there's always something else to pursue.
This year I did create a list of around 25-30 cards that I really want for my collection, so that's certainly focused my searching. My struggle has really been trying to keep focused on those cards, as well as being patient for those cards in the grades or eye appeal/quality that I desire.