What percentage of your lifts in training are misses? If you know an actual number, you're spending too much time on the math and not enough on the lifting, but if it's a big chunk, you're probably aware of it. I've talked a lot about practice and how important it is to practice lifting the way you want to lift even on the seemingly inconsequential sets or accessory lifts because that will be the majority of your training volume, and therefore the majority of your training time.

Your misses count as practice just as much as your makes do. Not only is it practice in the physical, technical sense, i.e. you're practicing the movement that produces a miss, but, more importantly, you're practicing mentally to miss. If missing is a regular occurrence in your training, it's a serious problem you need to address both through adjustments in your programming and in your approach to your training.

If you're constantly missing prescribed lifts, your prescription is off. Some misses are to be expected--if you never miss, you're probably not training hard enough--but this should be a very small number relative to your total training repetitions in a given workout or time period. Take a look at your program and ask yourself if you're missing a lot because you're prescribing (or being prescribed) weights/reps that are unreasonable. If so, quit beating your head against the wall and adust the program!

Missing can become a habit like anything else you practice. If you get accustomed to missing lifts, it becomes routine and suddenly it's not a big deal anymore. You may find yourself more than happy to simply miss and try to repeat the set to make up for it. This is fine in some cases, but making this a habit is training yourself to not be prepared and focused when necessary. Decide beforehand in a workout what you will accept as misses and repeats--don't get sucked into letting yourself repeat a set several times just to make it.

This article talks more about how to work with missed lifts in your training.