Slow Things Down.
Whenever you are playing from behind, the worst thing that you can do is, speed up your playing tempo (You don’t want to do that), what you want to do, is keep them out there as long as you can.
Your opponent, will want to finish you off fast and quick, so try to prevent that from happening, by taking more time in between points.
Place Each Point Hard.
The next thing you want to do after you slow things down is play each point as hard as you can.
Win or lose, breathe it away and play the next one even harder.
If you start doing this and keep at it, you will find your rhythm again and play yourself back into the match.
The idea here is to take it 1 point at a time and play that point on your terms.
No Unforced Errors.
Last thing that you need to do is make your opponent earn every point.
That means no unforced errors.
Before every point, picture in your mind what you want to do with it and then flow with whatever happens when the point starts.
Practice winning points in your mind more, before you actually play them.