A couple of days after buying some airline tickets with miles I found myself re-checking how much it would have cost to purchase them outright, and second guessing my decision of using miles to buy them. I mentioned to the boys that it was important to establish some criteria for when it made sense to second guess yourself, and when it didn't. In my case I consider:
1) Can I do something about it if I change my mind? No point in revisiting a decision about which nothing can be done, e.g., should I have gone to see that movie yesterday? On the other hand, if there is still time to do something, then it might be worthwhile to think about it. For example, A concluded he shouldn't have bought an iPad. He was able to resell and thus undo most of the "damage".
2) How significant was the decision? A major decision, such as where to go to school or where to go to work might deserve additional "analysis". But we shouldn't rethink minor things such as whether we picked the best shaving cream at the store the day before.
3) Finally, I try to avoid things that stress me, while devoting time to those that I enjoy.
The day after this conversation N came to me and told me he was second guessing his decision to cancel his WOW subscription the previos week. His friends were very disappointed he was not playing with them anymore. That was an easy one: after a bit of analysis he decided to reactive it.