Playing Online Chess seems to get rid of my bad blunders but after playing with little kids on a real match, I've made a very stupid blunder that decided the game.
Can't post the game, not recorded since It's a game played with kids.
You need to stop playing "Hope Chess" and start playing "Real Chess."
Dan Heisman gives the definition of Hope Chess as follows:
"HOPE" Chess - This is NOT when you make a move and hope your opponent doesn't see your threat. Instead, Hope chess is when you make a move and don't look at what your opponent might threaten on his next move, and whether you can meet that threat on your next move. Instead, you just wait until next move and see what he does, and then hope you can meet any threats. In my first 3 tournaments I played Hope chess and never won more than 1 game in any of the three. The speed at which you can play Hope chess also explains why I usually took only about half an hour for each game in these tournaments, even though the time control was 50 moves in 2 hours. Most high school level players play hope chess, but almost always lose when they run into a serious player who plays "Real Chess."
NM Heisman gives his simplified "Real Chess" thought process:
1) How the opponent's move changed the position and if threaten to do something positive.
2) What the goal/plan is for the position.
3) What the candidate moves are. A candidate move is a move that might do something positive, so you should at least look at it.
4) Which of the moves are safe. Omitting this step is Hope Chess.
5) Which of the safe candidates moves is best. At the end of this step, do a blunder check.
If you wait until the blunder check to make sure all responses to your move do not cause you to lose the game, this will limit your results. I used to play this way and could not get over 1300.
On every move you need to twice check for threats: 1) Once before you consider your candidate move. Imagine the opponent gets a second move. What threats would he launch: check? capture? better position?
2) Again after you have selected one or more candidate moves. Ask yourself, "Does my candidate move enable my opponent to check, capture or improve his position?".
Of course everyone blunders from time to time. It is, however, possible to strongly reduce the frequency of this happening. The best strategy is an additional safety check once you have decided upon a move. So, once you have picked a move, take a couple more seconds to check whether it does not hang a piece allow checkmate etc.
While this will take time and you have to force yourself to do it, initially, after a while you will see that you subconsciously do the blunder check without the need to actually force yourself to do it.
Oh, and of course another reason might be that you just play too quickly.
Playing a lot should reduce that problem. Of course, even then you must take time to look at your opponent's potential replies before you move. Even the least skilled opponent is making plans. The first thing to do is to make sure you haven't left any attacked unprotected pieces hanging. You must next also look for simple tactics against your pieces such as forks, pins and discovered attacks. That sounds like a lot of work but is necessary and becomes second nature with time. Be sure not to move too quickly. An old rule of thumb was to literally sit on your hands before moving to be sure you didn't move too hastily and superficially.