Re your pocket size - the standard and correct method( per diamond) to measure the corner pocket is from point ( where the cushion changes direction to angle into the pocket) to point. As you showed, you can use two balls(4.5 inches), but the actual centers of the balls is slightly under the cushion. Judging from what you showed, your pockets are at least 4.5” (pro cut), and maybe a little bigger. If you seriously want to become a better player, do not make the pockets bigger. Your pocket size is not too small - playing on bigger pockets will encourage sloppy ball pocketing. As was previously suggested, spend more time trying to hit center pocket.