Weightlifting is a very mental sport and success can be dependent on focus and attitude to remarkable degrees. Mental and testicular (or ovarian, if you prefer) fortitude is imperative not only for a given lift, but for a training session, a week, a cycle or a career.
The long term fortitude is usually thought of more as discipline. For example, the discipline to eat when, what and how much is necessary, the discipline to sleep adequately, the discipline to deny yourself activities or other things that interfere with your training, the discipline to sit in a bathtub full of ice in the middle of winter, the discipline to perform unglamorous and boring preparatory or rehab work.
When it comes to the fortitude necessary for training sessions, this is usually the will to not only get the work done, but to push yourself beyond what you feel you're capable of. There are many days you'll feel exhausted, weak, unmotivated, distracted or otherwise disinclined to train or to commit full effort and focus to training. These are the days you earn your progress, because these are the days that test your commitment. Everyone loves training when they feel good and perform well - doing what you're expected to do on these days is nothing special. It's the days you can barely force yourself to tie your shoes or put the bar on your back for the first time that define your character as an athlete. Are you going to surrender to complacency and comfort, or are you going to refocus on your goals and step up to the challenge?
Remember that on these days, the hardest part is usually getting started. You're stiff, you feel weak, you're moving slowly, your joints ache and your eyes are swollen with exhaustion. At this point, you're convinced there's no way you'll accomplish anything worthwhile in such a state. But if you can
get yourself moving, as slowly as necessary for as long as necessary, you'll invariably begin to feel better. Your body will warm, your joints will produce more synovial fluid, your nervous system will start firing a bit more quickly, and with all of these gradual improvements, your mental state will improve as well. No one ever gets worse. This is not to say that if you can get yourself to start moving, you'll always feel great. You may very well hate every moment of that training session. But you have to remind yourself that you don't have to enjoy every moment of training and competition - you only have to love more moments than you hate.
When it comes to individual lifts or sets, mental fortitude takes on a somewhat different shape. You need not only the will to perform the task, but the will to focus only on that task, forget everything else, even the set or lift that immediately preceded it. You can't go back and change a set or a lift you've already done; you have control only over the lifts you have yet to do, and any energy or focus directed to anything else only limits your ability to get the job done. One of the most difficult things a weightlifter can do is come back and make a lift after a miss. If you fail to control yourself, you'll be crippled by doubt and hesitation. You must find ways to forget what you've done and concern yourself only with what you will do.
Even successful lifts can create problems. Just as you can't allow a prior miss to prevent you from focusing on your next lift, you can't allow a prior make to prevent you from focusing on your next lift.
Confidence is a necessity for success, but confidence based on prior performance without focus and confidence on current performance can prevent adequate commitment to a lift.
Approach every rep as if it's your first and last.