Regardless of how good a player you are, you will always make mistakes. Doing so is inevitable. But whilst everyone is guaranteed to make a mistake at some point, it is the better players who learn from them rather than continuing to make them time and time again.
One of the most popular ways to learn from your mistakes, and indeed the mistakes of others, is to join one of the many poker forums available on the internet. Nearly all of these forums have sections dedicated to specific poker elements such as mid-stakes short-handed Hold'em, tournament strategy and pot-limit Omaha tips.-
Joining a forum is completely free in most cases and you will be discussing hands you have posted or have been posted by other members within minutes of registering for an account. This is one of the best, cheapest and most effective was to learn from your mistakes.
Another effective way to learn from mistakes, but one that is slightly more time consuming, is to go through your hand histories at the end of a session you have played. With many poker tracking programmes available to purchase for around $90 that will go through your hand history files for you, saving you time and tabulating them into a more readable format, there is no excuse not to start attempting to plug some holes in your game.
In order to progress as a poker player you simply have to first accept you will make mistakes, then learn not to dwell on them but instead let them make you a stronger person and poker player. Learning from your mistakes, so that you do not make similar ones in the future, is one of the most important Poker Lessons you can teach yourself.