Make sure you take it all the way -- opportunity cost is measured vs the alternatives you're not enjoying, but people often compare the actual vs the ideal: you break up w/ your girlfriend bc you're sick of the ways in which she's annoying, and you imagine not having to deal with that anymore. How freeing! But you don't factor in the flaws in her replacement, nor she with yours.

I lost a lot of time to this mistake. W/ girlfriends and a lot of other things, too.