It is a common held belief that the tennis forehand is by far the most important shot, and in order to be a tennis player of any standing you need to have this shot perfected to a T, and know how to use it in games to your advantage. You need to spend time perfecting the technique and then move on to using it from various points on the court, as you will need to modify it slightly depending on where you are on the court when the ball arrives.
You also need to consider the outcome of your tennis forehand shot. What do you want to have happen to the ball, where do you want it to go etc - are you trying to send the ball out of play and score the point, or do you need the ball back? This is part of a line up sequence so that you can send the ball out next time, whatever your motivation part of learning strong shots is being able to guarantee the outcome every single time.
Tennis players like Roger Federer have a very strong tennis forehand but this does not stop them having a bad day. It is always a possibility that one day you will just have a bad day, and no matter what, your once ideal tennis forehand will have stopped working for you. No matter what you do, you just won't seem to be able to make it work for you. The key here is to take a leaf out of Roger Federer's book and not panic. If he is having a seriously bad day for forehand perfection he simply adapts his play on court and accepts that he needs to use other strokes to compensate. Sooner or later the forehand will start to work for him again and he will be back on top form.
Stay calm, and if you have established that this is an off day, then leave it. Don't lose a game because you got obsessed with trying to fix your forehand. Adapt your game, use other shots, and focus on the goal of the game, not the missing forehand shot. This will only cause you to get more stressed and angry, which in turn means you will play worse and lose the game.
Even if you believe that the forehand shot is your worst and weakest shot, you need to try and change that mindset. There is nothing more destructive that the human mind, so if you tell yourself the shot is weak and awful, then guess what - you will play a weak and awful shot. Rather, you have to tell yourself that this is you current shot to work on, and practice. Accept constructive criticism and try again with fresh eyes. This is the most widely used shot, and it will be much better for your overall game if you can focus on improving your forehand rather than beating yourself up because you feel that this shot lets you down.