The mind is powerful. Put it to good use, and you can reach almost any goal you set. Kobe Bryant once said, 'It's not the number of hours you practice; it's the number of hours your mind is present during practice.' He was one of the greatest athletes of all time, and what he said is something you must memorize. His quote is more meaningful than almost any other quote you will find. I have seen so many players train hard and struggle, myself included, to overcome obstacles and break bad habits but make progress below par with potential because of a distracted mind. When hard training does not match the presence of mind, there is a disconnect between time spent training and progress. This disconnect exists only because the mind is not fully present during training. Your mind is the system in control of all your actions. So if it's occupied with thoughts, then it won't get a chance to learn deeply about what you're doing in the moment.
It's also not enough to know the rules of tennis technique and strategy. We apply that knowledge, and we must coach ourselves. The typical lesson consists of a coach giving you corrective advice, and you perform drills to work on their advice, which is normal and valuable. But students all too often do the drill and wait for the coach to provide more corrective advice without self-analysis. However, the way it should go is a player needs to stay aware and tune in to feedback from their shots ('inner feedback'), then use that feedback to make improvements. A coach's advice is external feedback. Think of this: only you have control over your actions and feel what happens. As experienced outside observers, coaches can see what you're doing and provide you with external feedback that you use to adjust your internal feedback.
The most important self-coaching rule is that you must develop skills through progressions. Any professional player you see on TV was at the same level as you in their journey to becoming a pro. No one skipped the training years to get to that level. You might see your favorite pro player running down 50 shots from end to end and pulling off a winner. If you try to do this without training to build up your skills, stamina, and focus, you will probably get discouraged from pushing yourself. That pro player invested a lot of time training to get there on an incremental basis. They began with the simplest exercises and gradually added new challenges as they improved. You must remind yourself that the journey brings you to the destination so you don't get caught up with skipping steps or rushing yourself.
So, the takeaway should be twofold. First, you're always engaged with what you're doing. Your mind must not drift into other thoughts. Second, you use internal and external feedback to make adjustments, corrections, or emphasize something that works well. Then you follow the rule of incremental progress. This will keep you from forcing it and pushing yourself to think of finish-line results.