For the voice of doubt, you must build faith in yourself.
It's not about aiming for perfection, after all, we can never be perfect. But you can be a good player, set out to do your best, and have faith in yourself and in your shot selections.
Building faith is easier said than done, but with persistance faith will improve results and results will improve faith. In time you'll get there.
Expecting perfection can destroy faith, however. We are all certain to make some mistakes, accept this and when they occur try to learn from them. Retain perspective and realise that overall, you are a good player.
Inconsistancy can rear its ugly head in three key forms:
i) Missing easy shots as your concentration wanders
ii) Missing pressure shots, the pressure distracts you from your game
iii) Missing 'normal' shots in open play that you can sometimes make perfectly, but at other times it just doesn't seem to happen.
Outlined like this, it's clear that concentration is one crucial element. Concentration is a skill you can build, with practice. But concentration will come much more easily if you're genuinely eager to be playing pool - if you find yourself playing simply out of habit or force yourself to practice, concentration will be much harder as the mind doesn't genuinely want to be playing.
You can alleviate pressure and focus more on easy shots by accepting one important principle: Every shot, every ball, is important, and should be treated as such.
In 8-ball, you must pot all 8 to win, in that respect the 8 is no more special than the 5, and that easy 12 has to be made in order for you to progress to victory.
Things are slightly different in 9-ball, but nevertheless every individual shot can be instrumental in winning or losing the game, whether it's the 3 that is an early step in your run-out, or the final 9, every one must be made to complete the out.
Learn to treat each ball with equal respect, and you will release unnecessary pressure from yourself, and keep the mind focussed on getting sitters into pockets.
You can also help consistancy with your shooting routine - people have already mentioned pre-shot routine, that's one aspect, but it also goes into shooting routine too.
If you are following a standard routine used for every shot you force yourself to concentrate, and cut out lots of variables that would otherwise sneak into your shooting causing inconsistancies.
Personally, I use a set aiming routine, then when down on the shot feather the cue ball once, look up to the object ball, and stroke. Same every time.
(With the exception that where awkward cueing is involved I look up to the OB, then back to the CB, then stroke.)
It is important to thoroughly train yourself in performing your chosen routine, so that in match play you can do it automatically, without requiring conscious control - so you don't need to prompt yourself to perform each step. You may wish to take a little time away from the table to go through your routine, imagining yourself playing the shot, so that your brain has a feel for the routine, learning precisely how to perform it.
