This is avery vague and broad question, full of naive hopes and expectations.
1, The main way to get better is through hardwork and practice.
2, Work at improving all aspects of your game. If you have weaknesses, they will not be secret for long, and your opponents will take advantage of them.
3. If you work hard and practice, your game will improve and you will, therefore, become stronger mentally, because you will have confidence in your game and your strokes.
4. You know you are improving when you no longer have to think about your next move. Your strokes and placement choices will come without conscious effort.
5. Find a book of practice drills and practice the patterns until they become natural. You can also find drills and coaching ideas on various internet sites and youtube.
You ask about what you should "think" about in various situations. In a match, during actual play, you should do very little thinking. Do your thinking when you practice, or between points, if you have to-- Imagine you are a samurai, in a battle situation- once you stop to think, you are doomed.