It is important to always keep a positive attitude even when you are losing.  In my short table tennis career, I have seen many players beat themselves as they gave up when losing or beat themselves up after playing a poor point.  Everybody makes mistakes in their strategy and/or technique during matches, even the most elite players in the world.  I have made comments to younger players that were losing to stay positive and heard back “Positive?  This guy is killing me!  I have no chance.”  I often say to myself, “This isn’t over yet.  I can do it.”
During 2013 I had two memorable wins against higher rated players when being down 2 games to none and losing them badly.   In one match, I lost the first two games by 11-6 and 11-2.  In the third game, I went down 7-1 and called a timeout.  Now, this was really too late for a timeout, but my mental state earlier was such that I was losing so badly, why bother?  What a mistake.  After going down 7-1, I thought “wow, I need to fight for this even if I lose”.  How did I turn it around?  I thought come on, let’s go for it.  How is he beating me and what can I change?  Take it one point at a time and do not get caught up in the game score.  Keep a positive outlook.  It’s great practice if nothing else.  And finally, loosen up.   In the other come from behind win, I lost the first two games by 11-2 and 11-4.  I won the next three games by keeping positive and making some small changes.
Some of the changes that I use are serving long instead of short, flipping my opponents serve  instead of pushing short, and stepping back a few inches on my opponents serve in order to read the spin better and give myself an extra fraction of a second to return the serve. There are a myriad of strategies that can be tweaked to start changing the outcome of points.
If I had given up in these matches, I probably would have continued to use the same strategy and lost the match.  This can then carry over to the next match. 
Just as important is to get over a loss as quickly as possible.  Keep a positive attitude about the loss and what you learned from it.  Quickly analyse that match and start to prepare for your next match and how you are going to win it!  I lost a close match to a player rated about 150 points higher than me this year and then thought about it the whole time I was losing to a player rated 250 points lower than me.
As Winston Churchill said, “We will never surrender.”