Hi Shaky,
the camera idea is really a good idea. I myself just received a tripod for my camera and planning to video tape myself.
you can also send videos of yourself to some qualified instructors who can evaluate you further. most of my analysis I did by myself and some might apply only to me, but I feel that others may have variations of the same problems.
I completed the puzzle pieces by reading online advice from some instructors and also I had a few instructors in real life.
about keeping back arm straight. making a video of yourself will help you see the problems and help you a lot, perhaps an instructor can see more things in the video if you send it to them.
but when you get down you don't see your back stroke as you said. so a device like that laser stroke are probably a good idea because you actually know how it "feels" to have it straight.
may i suggest also an exercise which is used by a lot of europeans , you simply place a medium to long cross table shot, then first follow the cue ball to the pocket, then second exercise , stop the cue ball all at medium speed. while doing that practice, think of making sure your back arm goes straight back, that you are aligned and that your body is balanced and legs firm on ground. you will see it easily on this type of practice and it can make your back swing straighter, i think.
i heard that niels feijen did that for some time. he's straight as an arrow now.
about concentration to keep on target, how to keep your eyes longer on the target? one advice I read, is this: the time you take to look at the object ball before stroke delivery, double it. to lock your eyes on the target a bit longer than you would think you need to, this is scientifically proven in many sports (from the articles i've read). that is probably because of the way the brain works...
about nervousness, and concentration, yes the camera might help also to see what you're doing.
to truely improve it though, may i suggest something that might seem silly, which in fact improved my concentration by 200% which is simple meditation and/or some breathing techniques. in fact after doing this for a few days, my brain and eyes naturally found out that i was not looking at the target long enough, then naturally i would find myself more concentrated on the target.
the reason to have tension and lose concentration i think is because of too many thoughts and emotions inside, and also you are trying to think to much and execute less. to reduce that, meditating 2-3 times a week for 20 mins a session should do some good.. if you have the guts to do it..
if you don't know how to do it, i would suggest look up dr. chopra on youtube he explains how to do meditation well , or just look up anything else you wish or go to some classes.
breathing exercises also work, like tai chi or yoga breathing, etc.
also another reason of nervousness is thinking too much about the other player's game and about the other things around you, including the behaviour of people, the music, weather, etc.
you find that even when you concentrate not to think too much, it won't really work, because you are thinking about not thinking. the only way is to reduce those thoughts. and this can be accomplished by breathing exercises, or meditation.
it's also known that breathing is very closely associated with thought process. the slower your breathing the less thoughts you will have.
also when you get down on the shot, if you breath in, and hold it, and not breath out, you are keeping the co2 from coming out so your brain and head becomes quite tense after some time.
there is probably a lot more to sports psychology , but i find it's quite important. the mental thing is like 90% of pool. wouldn't you agree?
anyhow i don't want to hijack this thread into something else than it's meant to be ( posting 14.1 results and videos

) so i suggest you work on those things if you wish to, then perhaps read more about sports psychology or check with a sports psychologist if you want to help eliminate some mental things from yourself.
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