Respect the Pressure
I have a weak mental game and I often "bottle" big points with unforced errors, and generally my game is not as good in pressure situations. Any tips on how to improve this?
First, you need to understand that pressure is very real. The difference between winning a point in practice and in match 40-Ad is very different. It's like saying playing an instrument in practice is the same as playing on stage. You simply need to respect the difference that pressure and stakes put onto a skill. The emphasis on respect is very important. Pressure breaks down all major skills and your tennis will inevitably be affected. But it works that way for both players. Usually it comes down to who's game is more susceptible that day. Look at Roger Federer this Wimbledon. Sometimes the pressure can take you beyond your limits. Wawrinka comes to mind during his grand slam runs. If you can respect what pressure is, then you are on your way to playing better in tough situations. Getting over the guilt over losing tough points, and understanding the effects of pressure rationally makes you play and understand the situation better/confidently.
Focus.
The next step is understanding "how" to play better in these situations. Playing tough situations comes down to focus. That's it.
If you are worrying about the score or your shot you have lost focus. If you are angry or emotional you have lost focus. If you are "thinking" about your game or shot, you have lost focus. Focus is just pure execution. Nothing else. It's like answering your multiplication tables. 2 x 2 is 4. No thinking - just pure response. No "uh..." just "4" . This is what is should be for your tennis. You aren't distracted by your circumstances, your shots are just pure muscle memory.
Focus is therefore a skill and needs to be honed like anything else. Skills are honed by being consistent and making practice into habit. If you are constantly worked up criticizing yourself and interfering in your shots then you are well on your way to an inconsistent mental game. If you have to think about moving your feet in game you are also already too late. It should be ingrained that you don't have to think about it. Being confident and automatic in all the actions and shots you do makes it easier to stay focused and should be big goals in your practice sessions.