Reddit mentions of Vic Braden's Mental Tennis: How to Psych Yourself to a Winning Game

Not sure if this will help short term... Being mentally strong is a skill, and you can gradually learn it, just like you can learn to hit a serve.

Subscribe to the Essential Tennis podcast and listen to their old podcasts on the mental side of the game. They are not available on the website, but if you use a podcast app, you will find them. The casts are really good and will give you a very clear idea what you are battling with.

Also, there is a vast number of blogs, articles, books on the mental side of tennis. Here are three of them:

https://www.amazon.com/Inner-Game-Tennis-Classic-Performance/dp/0679778314/ref=sr_1_1?ie=UTF8&qid=1526928034&sr=8-1&keywords=mental+tennis

https://www.amazon.com/Vic-Bradens-Mental-Tennis-Yourself/dp/0316105171/ref=sr_1_2?ie=UTF8&qid=1526928034&sr=8-2&keywords=mental+tennis

https://www.amazon.com/Winning-Ugly-Mental-Warfare-Tennis-Lessons/dp/067188400X/ref=sr_1_3?ie=UTF8&qid=1526928034&sr=8-3&keywords=mental+tennis

Here's another book to try out

http://www.amazon.com/Vic-Bradens-Mental-Tennis-Yourself/dp/0316105171