The match score is simple. It is the best two out of three sets. So if you win the first two sets, you win. If you win the first and third set, you win. If you win the second and third set, you win.
Win the first set!
In any match, it is pretty rare for someone to lose the first set and then win the next two. Usually the conditions involve the winner discovering some sort of huge weakness in their opponent late in the first set. An injury occurs to the opponent (or reoccurs). Or the opponent just gets really tired.
However, if one of those things does not happen, the person who wins the first set, most likely wins the match. The reason being that the opponent must win two sets in a row to win after already played a set. Winning the second set is hard mentally and physically. But to keep that going without a losing it in the third set shows a tremendous will to compete.
Tennis is a marathon
Every player has a mind, body and spirit. A mind to play a good mental game. A body to play a good physical game, and a spirit that gives you the will to win. When the player's energy is low in one of these departments, then it all falls apart. You have a certain amount of energy to give for all three. You allocate your energy in order to play the best game you can. This is constant throughout the match. If you almost twist your ankle, you add more energy into physical and spirit. It is on break point, more energy for mental and spirit. Your forehand is not working as well as usual, mental for the adjustments, physical and spirit for the execution.
Ideally, you would want to train enough to not have to worry about these things. however, not many people are good at training all three parts (nor do they have the time). Fortunately, people that play on your level have the same problem.