Our thoughts pervade our every action throughout the day; your every muscle movement, breath, creative idea, or motive to procrastinate has a thought, or more, that drives the action forward. Even thoughts that you don't think cause action will provoke you to act involuntarily; your heart rate may increase, your eyes may shift toward a certain object, your brow may furrow, or a completely different reaction may occur.
As a part of your mental training, you must learn to control the thoughts that enter your conscious and subconscious minds. Stop yourself from being, or don't allow yourself to become, a victim of your own thoughts.
Have you ever psyched yourself out before a performance? The performance could be singing in public for the first time or lifting a heavy weight in a gym. In either case, your thoughts affect your success. Having a feeling of butterflies, or that your throat is closing, or that you can't control your breath will affect your singing because of your apprehension or fear. The same applies to weight training.
Picture this scenario: You are exercising in a weight room and, after considerable warming up, you are ready to perform a heavy bench press. You think that you're strong enough to lift it, but the thought of not being able to fuses with your thoughts about how heavy the weight is. You get on the bench, ready to lift the weight, spotter behind you prepared to assist you. Your foot starts tapping out of nowhere; you start to look everywhere and nowhere in particular; you wriggle your fingers a couple of times on the bar. As you lift the weight, you struggle through it, telling yourself you can do it, but every part of your body seems to fidget. Nevertheless, you successfully lift the weight and feel rather satisfied afterwards.
This scenario is the result of negative thoughts. You've told yourself how difficult lifting the weight would be. Your body likes homeostasis (to remain just the way it is), so it IMMEDIATELY stresses out and looks for any possible way out of doing this hard thing. You looking all around with your body fidgeting is you mentally trying to escape from what you are doing. You do it successfully, but after the stress that your body has gone through, your endurance is incredibly reduced.
The important thing is that even in that stressful state, YOU LIFTED THE WEIGHT. You somehow told yourself that you'd make it through, and did so. Imagine how much easier the weight would move if you started the lift with that mindset. Instead of starting the lift thinking you can lift it, know that you can lift it. You might even surprise yourself by lifting things that you believed were too ridiculous for you to manage. Often times, your success is as simple as having the mindset of success. If you're going to have thoughts toward your actions regardless, train yourself to make those thoughts positive and confident. They'll force your actions to that success and confidence more often than not.