Some players have reputation as being "winners" because they seem to be able to pull out close matches. There are two aspects to this. One is mental – nervous players don't do well in close matches. The other aspect is tactical – you need to learn what to do tactically in a "Big Point" near the end of a game.
This means being aware of what has worked, and what hasn't worked, up until that point. Many players find a successful tactic, but don't think to use it when the game is on the line. Or they use a tactic that hasn't worked. You probably aren't going to keep track of exactly how many times each tactic worked or didn't work, but you have to develop a feel for it.
If you have a serve, receive, or rallying shot that really gave the opponent trouble, there's often this tendency to not use it at a key point, since you figure the opponent is expecting it. That's over-thinking - overwhelmingly it's better at key points to go with what was working. It's just as likely your opponent is thinking you will cross him up with something different! If you don't, say, use that serve that your opponent kept missing, you may end up losing, with your opponent perhaps asking afterwards, "I'm curious why, at the end, you didn't use that serve I couldn't return?"