You've just lost a point. Perhaps a big one. Does it matter? Of course it does! But does worrying or thinking about it help? Not at all, other than registering the tactics that worked or did not work as new data points. As far as you are concerned, the only thing in the world that now matters is the next point. That means thinking tactically and preparing yourself mentally for that next point. And then, as soon as that point is over, guess what? Repeat. The next point is the only thing in the world that matters. Prepare yourself tactically and mentally for it.
How best to do this? You can't think about two things at once, so if you think tactically between points, that's what you'll be thinking about. Don't overthink - just go over the most important things, such as what serve to use, and what receives, shots, placements, etc. to favor. Then, before the point is played, clear your mind and prepare for the next point. (Thinking tactically between points allows your subconscious to reflexively do those tactical things during points.)
Note that Jan-Ove Waldner, often considered the greatest player in history (well, before Ma Long) considered this his strongest attribute, the ability to completely focus on the next point, to the exclusion of anything else.