Let's face it, we've all been there. Tapped for a different color than you should have, shuffled your hand into your deck, blocked a creature you didn't realize had deathtouch, missed an entry or death trigger. The list goes on and on. As the saying goes, "mistakes were made". No matter what the game is, someone out there is making a misplay. What's the best approach when that someone is you?
The first thing to ask yourself is can you recover? If the answer is yes, then just shrug it off and keep playing. Mistakes are ultimately part of the game and everyone will make them at some point. Your opponent may not even be aware of it. On the other hand, your opponent may be the one to identify and correct it. Both players (in a head to head match) have some level of responsibility in maintaining game integrity.
When you're new, misplays will be a certainty. One good way to gauge your improvement is noticing fewer misplays in your games. When you are in casual events and matches, never be afraid to ask a question if you're unsure about something. Store staff, judges, and even your opponents (in certain situations) should always be willing to help you. Even the long time players remember they were the "newbie" at one point. Nothing is better for a player base than new players who want to learn and be a part of the community.
On the minor side, misplays can cost you a card you could have drawn. On the major side though, misplays can cost you the game or a spot in the top cut. The toughest part is that they have a lingering effect. They can cause you to be frustrated and lose focus on future plays, which may result in another mistake later in the game. They can cause you to concede (scoop it up) because you feel there's no recourse for you. They also can be the best teacher. Some of the best lessons taught are from failure if you're willing to learn. They will help you realize what you should or shouldn't do the next time you're in that situation. They will help you realize if your opponent has made a mistake that can benefit you.
When you misplay, try your best to not be negative, especially to opponents or anyone else with you. The longer you let it effect you, the tougher the game is going to be for you. It also stops being fun which is the goal the vast majority of the times you play TCG's. Outside of major events or money prize tournaments, most of the times you are at your LGS you are playing with the regulars for some fun and some packs. Do the best you can to laugh it off and see if you can get out of the mess you put yourself in. You may just surprise yourself.
In the moment, misplays are frustrating and annoying. In the long term, misplays are the necessary growing pains and battle scars you need in order to become a better player. So the next time you see someone sighing heavily, slapping their forehead, or doing a dramatic head sag from a misplay, you can feel a little better about yourself for all the times in the past when that someone was you.