It's not uncommon for players to be up a set only to have the tide turn and eventually lose the match.
Losing your lead doesn't have to result in your losing the match. The only thing you can do is to be aware of the psychological factors that may cause you to lose your lead and try to make them work to your advantage.
Don't Relax After a Lead
The so called letdown is a natural response of our body and mind to relax and rest.
You actually have to consciously fight this response in order to get back to your optimal level of play.
Know beforehand that a letdown will happen after each set and raise your activation level back to your optimal state.
You do that by making a few jumps, pump yourself up with power words ("Come on," "Let's go," etc.) and breathe a little bit faster.
You don't want to lose this opportunity
It's interesting, how our mind works.
When we have something, we don't want to lose it. But it is exactly this feeling that causes the loss.
Not wanting to lose this opportunity causes you to play safer, more cautious, you don't take the chance when you have to, your body starts tensing and you cannot hit relaxed, powerful shots.
And when you play someone close to your level, this type of play is most of the times not good enough to beat them.
You need to accept the fact the even when you play your best, you will lose a lead sometimes. That's sport.
Become aware of this "not wanting to lose" thinking or feeling and let it go. This type of thinking will not help you.
Dismiss it and remember in what mental state you were to have won the previous sets. Get back into that state and fight.
Your opponent now has nothing to lose and goes for the shots
When you put someone against the wall, they cannot back up any more.
Good players will dig deep and start fighting back. There is nothing to lose and fighters want to go down fighting if they have to.
They would rather lose missing their shots than be humiliated by their opponent's winners. Read this sentence again and again.
This is how you need to play and this is what you need to be aware when you have a big lead against a good player. You need to play more aggressively: play with your backhand, approach the net and serve better. Play like you have nothing to lose, no pressure.
You have to try and keep your opponent down when they are down.
Obviously this is NOT under your control but trying to do this IS within your control.