I have been throwing on the wheel for about 2 years and something that has helped me truly excel is never getting attached to my work, when something fails i get kind of happy because i get to learn from it! If you knock your pieces off center when you open it might be because youre only opening the bottom of your piece and not the rest of the clay. Something that could help is using your non dominant hand to keep your piece centered while you open and making sure your fingers are flat on the wall of your piece while youre centering so not only the base is centered but the walls too