I would say start using the tactics trainer on this site---turn off the tactics clock so that you don't have to hurry---set the lowest level tactics at 600-650 or so because it sounds like you need very basic tactics---I think you find the tactics editor under "home" and then "account". MAYBE that will help---maybe, maybe, maybe; it shouldn't hurt your skill level.
GOOD LUCK!
Just thinking...
Look for several candidate moves
Did your opponent make a threat with his last move. How does the game look for him?
Play a lot of games
This has nothing to do with tactics or strategy or concentration.
This is just confidence.
Just get more confidence by playing more games. If you keep at chess this problem will dissapear. Grandmasters don't get this because they have played chess for years and have played tens of thousands of games and are very comfortable at the chess board.
The chess board is still new to you. I have been playing chess for a year and a half and still get this, though not as much as I once did.
You have to find your own pregame routine to fix your confidence and concentration. It doesn't really matter how good you are at tactics if you are scared of the chessboard and always wonder what's going to happen next.
I'd recommend playing a couple games against Computer1-EASY. The computer is programmed to play some pretty bad moves of its own, but if you drop a piece it will almost certainly take it, and if you allow a forced mate it will always find it. Overall it's pretty consistent at punishing obvious errors. In addition, if you play solidly and manage to capitalize on one or two of the computer's errors, you will probably win the game, so it's a great confidence (and rating) booster.
I just started playing chess and I noticed that I always move pieces that I regret moving right after. Like, I will move a piece and then the instant I release the mouse button I have an "oh shit" moment and realize I just made defenses vulnerable, or that there was a much much better option. It's always right after I move it that I realize how dumb it was, I don't see it ahead of time. It happens a lot and I just lost a game that I was winning because I accidentally compromised my defense with a single bad move. Is here a way to overcome this? Thanks