Perhaps it is your body/head/feet alignment and NOT your stroke. This cannot be diagnosed over the internet, but if you get with an instructor that know the difference it could save you a lot of trouble. Stroke mechanics are different for every player, so is body alignment, balance, etc. There is also the case where you could be twisting the cue by having the wrong grip.
Usually I have my students remove their thumb completely from the grip and try stroking with just the 3 fingers (ring, middle, forefinger) - in the thumb's absence you will see some wonderful action applied to the cue ball. This is because we have muscles that connect the thumb, wrist and forearm... the major muscle is called the Flexor Pollicis Longis- which is also connected to other tendons in the wrist and the forearm.
In this diagram you can see the deep muscles and tendons that you would use to perform your stroke. As you can clearly see, the Flexor Pollicis Longis moves in a different direction than the other muscles. Try eliminating your thumb from the grip for a few shots and see what happens. Afterwards, gently return your thumb loosely back into the grip. I think you will be surprised at the difference.