Athletes don't wake up one day with mental toughness, and it doesn't happen by osmosis. As I have stated, mental toughness is a progression and is learned.
1. Proper bullpen work! Proper bullpen work starts with having a plan. Each time a pitcher takes the mound, he should have a plan! Whether in a game or pitching at practice, ask yourself "what am I working on today." Plan the work and work the Plan!
2. Read. Find topics that you like to read and find time to read everyday. Some may question this but it has been proven to increase focus. The ability to focus in difficult situations is a must! Just like you prepare physically by lifting and throwing, pitchers need to prepare mentally by reading to increase focus. Increase you reading time just like you increase your weight lifting or your running.
3. Visualization. Let your mind become the practice field for what you want to accomplish during the next game. This practice field is always available. For 5 minutes everyday, see yourself throwing strikes. See yourself in difficult situations. Get away from everyone for 5 minutes, close your eyes and see yourself locate your fastballs down and away. Baseball is a kids game, it should be fun! Use your imagination and see yourself being successful. If you need a little stronger picture to your imagination, go through your delivery with your eyes closed and see yourself locating the baseball down in the zone.
Do something everyday to make yourself better!