It has happened to everybody. You play a seemingly beautiful move and your opponent drops a capture, check, pin or fork on you that you overlooked. Your first reaction is "$%#^&%!" and your second reaction is "how in the world did I miss that?".
This is a frequent occurrence for many lower class players and occasionally happens to upper class (B/A/Expert) players.
Here is an exercise to get your mind used to looking for these kinds of moves right off the bat (after all, they are basic tactics and you need to see these first and foremost in a position):
Take any GM game and pick a side. Begin to replay the game one move at a time. After each move, write down all the captures, checks, pins and forks that can be made as a next move. Record:
Pins: Bf5/R/Q (Bishop to f5 pinning Rook to Queen)
Forks: Nf5/K/Q (Knight to f5 forking King and Queen)
Usually you will be amazed how many of these moves need to be accounted for. The benefit of this exercise is to give your mind capture/check/pin/fork pattern recognition. It is also somewhat of a chess vision drill as well.
Try this for several games. It may take a bit of time off your studying, but if you have a problem missing these moves in your mind's eye, this will help you out in a big way.
As usual, keep the comments coming and all opinions are welcome!