People talk about “the mental side of the game” in many sports.
Two things from my earlier years showed me how important the mental side really was.
- Full-contact kickboxing/karate
- Night time parachute jumps
You had to be mentally prepared for those in order to act without hesitation when the time came.
Fast forward to strength training. It is very important to be clear-headed and completely focused on each rep you do.
The common enemies people have in their heads are:
- lack of confidence
- fear of getting under a heavy load
- distracting thoughts
- an out of control ego
- over-thinking aka paralysis of analysis
- lack of experience
- knowing your technique is not that good yet
All of these are bad enough individually and worse when there are more than one affecting you. They can cause you to miss the lift, get injured, or even killed.
Get them out of your head!
I am sure there are some I have missed, so feel free to post them in the comments section.
The Cave 
I agree in your description of “enemies” for these ways of thinking. Comparing myself to others sometimes can affect my decision-making. I want to consistently battle these enemies so they occupy a place under my foot more often than in my head!
Josh–We can be transformed by the renewing of our minds in Christ Jesus.