Don't think of the black cat. Despite telling you not to think of the black cat, an image of it likely popped into your head anyway. In training for tennis or sports in general, when you think of not doing something, your mind ends up thinking of the action you are trying to avoid. Thus, you must avoid the "don't do this and that" rhetoric for yourself and focus on DO! Doing is goal-specific, and "don't do" is task avoidance. When you get preoccupied with avoiding a task, you sap your mental resources and willpower without getting the benefit of experiencing the correct action.
The semantics of "don't do" has a different effect from "do" even when the result appears the same. Suppose you must cross to the other side of a creek on a makeshift narrow bridge built from planks. There are no railings, and it's easy to fall if you're not focused. In the first scenario, either you or someone tells you "Don't fall." You stress out, and as you're crossing, you keep repeating "Don't fall." Your mental resources are now being spent on thinking about not falling. It's an action you're trying to avoid, but you're still thinking of that action while simultaneously trying to perform. Essentially, you're moving while thinking about falling. As you're thinking about falling, you have fewer mental resources to dedicate to walking accurately on that makeshift bridge. Your likelihood of an accident thus increases substantially. Now, assume that you're not telling yourself anything. You simply focus and begin to walk. There are no thoughts about falling, only an understanding that you must cross. Suddenly, your mind is clear and your resources are utilized to walk on the makeshift bridge. In the second scenario, your chances of falling are low, if not impossible, because you're zoned in on the task at hand.
The gist should be clear at this point; when you spend energy on avoidance, you're taking away resources from the action you want to take. In sports, it's very easy to focus on avoidance, thinking you're going to improve. Don't miss, don't move a certain way, don't swing too big, don't move slowly, don't stand straight -- the list can be endless. But are you learning anything? Are you gaining experience by following "don't do" orders? All this is paralyzing, stressful, and saps your willpower.
So, make it a rule that you will have a clear objective for the action you want to take. Train yourself to find solutions through positive actions and not by avoiding failures or bad actions. Make more shots, move a certain way, take compact swings, move aggressively, keep your knees bent. If there is a skill you need to learn or a result you want, focus on the action and let any fear of failure slide away on its own. By default, positive instructions will keep you from doing the things you don't want to do. If I am focused on moving, which is an essential part of tennis, then I won't be standing around.
To help you develop a positive training mindset, follow these high-level rules:
Accept and embrace failure. It's the only way you will learn new skills. Fear of failure drives us to action avoidance.
Believe in yourself. If other people can learn to do it, you can.
Take your time and work consistently.
Learn to love the process.
Finally, you might not get what you want, even if you do everything right. Regardless of the outcome, the skills you obtain from training will translate to other aspects of your life. Your athletic mindset will bring you strength that will follow you along any path.