Getting mentally tougher is easy if you are willing to pay the price.
Do you really want to be mentally tough? Get uncomfortable! Put yourself in difficult situations over and over until you learn how to remain focused, calm and intense under adverse circumstances. Seek opportunities to fight when in situation you dread.
Here are a few ideas that will provide you with the exact setting you need. Just remember that for the exercise to work, you need to make sure you find an opponent that is about the same level as you and you need to try your best to win.
Choose the drill you dislike the most and do it over and over.
Play matches against players you hate to play against.
Play matches in the wind, the windier the better.
Play matches with a tactic that you are uncomfortable with and stick to it regardless of the score.
Play matches after you worked out, when you are really tired.
Play matches changing to a total different racquet every 5 games.
Play matches where all your shots landing on the line are out.
Play matches during the hottest part of the day on a very hot day.
Play matches on the worst court you can find. The more cracks the better.
Play matches with different balls, one very old one, one old one and one new one.
Etc. - Add any situation you dread
To increase the benefit of this type of practice, bet something slightly painful in each of your matches.
Once you stop fighting the different situations and accept the challenge, you will find yourself well on your way to a whole new level of mental strength.
No pain no gain!