I think it's a bit stupid and self-destructive to regret about time spent with a girl (and money spent to the boot). Good times were had, money spent are money that you won't get back so I don't see any sound rational reason to regret about this unless you like to feel miserable and hate yourself.
Oh ease up. It all falls into getting some distance in your mind between you and the ex-gf. It helps get you to the "its over, time for me to find someone better" stage.
Obviously taken to the extreme, its bad. But a little "ugh, what was I thinking, better find someone better" is going to help.