A lot of clients who engage in 1-on-1 training do so because they feel like they can "shut off" or "stop thinking for an hour." You've been busy all day, ramming around, the one hour you have to relax is your gym session where (at least at our facility) you get directed what to do, weights changed for you, programs are written out for you - a pretty cushy situation. However, your mentality in the gym is a big factor in succeeding in your goals. How you approach each workout and each lift matters. There are three major components to this that I have observed over the years: focus, confidence, and aggression.
Focus: Which muscle groups are working? Why? Are you performing the motion correctly? Is your form where it needs to be? Is there pain? Does it feel good? All of these questions should be running through your head while you are lifting. As a Coach, I can look at you from the outside and identify movement patterns. I don't have access to your head. This is what we call biofeedback; listening to your body and how it responds. If you're doing squats and they look fine from my POV, I'm going to assume they feel okay. I can't tell if you have pain in your knee or in your low back (although, I can assume) or you're not feeling the intended muscle groups. This is YOUR job. Coach and client are a team and we need to work as one. If you come into the gym and shut down, the risk of injury goes up and you will not progress as fast to your goals.
Confidence: I see this all the time; "That looks heavy," "I can't do that weight," "Are you sure you have the right numbers?" You've already lost. You're in your head. The SECOND that doubt creeps into your gameplan, you've lost. Should you be nervous? Sure, especially if it is a heavy weight. But, there should be absolutely no doubt in your mind that you will get that rep. Exert confidence in your lifts and not only will you see better results but that confidence will also translate to real life. You'll notice situations might not be as daunting or scary as you made them out to be.
Aggression: Aggression gets a negative connotation. Yes, it can be used in the wrong way... but channeled into a focused exertion of energy is a major factor in weight lifting. That's one reason we play Metallica and Five Finger Death Punch and not Barry Manilow. Lifting is a very primal activity. You need to be able to access that part of your psyche, especially when you are going for heavy reps. Focus (see, focus!) on channeling your aggression into performing your lifts with maximum intensity. Aggressive BUT controlled.
Lifting is just as much mental as it is physical, I would argue it's probably more important. If you walk into the gym with a shitty mindset and not ready to go, you can assume the workout probably won't be as good as you intended.
Bring the aggression.