when i play by myself (or with a close friend), i can run 1-2 racks out of 10 regularly. when i play my local tournament, i seriously have a hard time running 3-4 balls on easy layouts. i know im capable of doing sooo much better just by the way i play when i dont play tournaments. its all because of my nerves. how do you handle your nerves? my cueing hand starts shaking when i get nervous and its literally impossible to make an accurate shot.
Pool is extremely difficult sport to master, unless you practice 3-4 hours a day. There is no sport that have that many variable and all have to be right on to perform the shot. Some people will tell you just take it easy, let your subconscious take over; that is very bad advise for c,b and A players, until you are a pro, you have to think and take it slowly, sure some shots are easier than others, but shots that are hard cut, speed, position requires a check list before you go down, and a check list after you go down, and a small check list when you are about to fire.
You said when you practice you shoot good, and in game you shoot bad, sure different table (maybe), cloth, different set of balls, different CB, conditions all matter A LOT, and worst of all your break, balls might not be as easily spread as you normally like them. Lets say you mastered ball pocketing, mastering position/patterns, kicks, banks play is where experience matters
Keep playing, get into every tournament or gamble you will get good. If you think it is medical problem seek a doctor and there is great medication for it.
Best of luck.