We wouldn't really learn anything if we never made any wrong decisions.
What ever you decide for what ever situation, as long as you made the best decision possible with a cool mind, you really shouldn't have any regrets.
Sometimes we learn, even after we tried out best that our decision was wrong. Depending under the circumstances, i.e was it harmful to someone, will it become harmful, it clearly wasn't the right decision for what ever reason, etc ... one can try to rectify the situation or the decision, if possible. Thats the best you can really do.
If however, the decision was wrong and you've come to realise but don't really care about rectifying it, then I guess you just learn and walk away.
I also think the worst decision we tend to make ( or at least me) are hasty decisions or decisions I make when I'm in state of great anger
