Once again, I agree with our pal BU-Tennis.
One of the most useful mental disciplines in tennis can be learning to forget the overall score and get comfortable with planning your attack for each individual point. Take a second before the start of each point, whether you're serving or receiving, and revisit a simple plan in your head in terms of how you want to at least start the point to try and get control of it. You might decide to do the exact same thing for several consecutive points, but this ritual can be good for keeping you sharp and "in the moment". When the action starts, you'll already know what you want to do.
This sounds simple, but it can be mentally demanding and you may need to practice this ritual by playing tie-breakers during practice grinds. Do a mental check-in on your plan before each point. As you get used to this method, it can make the overall match score have less of an effect on your energy and outlook. It's usually much easier to concentrate on winning a point instead of an entire match. Learn to only win the point... lather, rinse, repeat.