Jimmy Reid discusses positive mentality on one of his instructional tapes, and it helped me out a lot. I guess you could call this attitude, or at least an approach to the game.
Reid calls it the "If" theory, meaning you tell yourself "IF I make this ball and land here than I'll be able to take this next shot, IF I make that I should be able to get out". This is a kind of thinking that sets you up for positive action but doesn't lead to game and spirit shattering negativity should you fail to execute your shots as planned.
Your thought process will be aware that there's a chance you may not succeed. A miss becomes just a miss and poor shape is just an opportunity to show what you're really made of. If you get out of shape you start over again and always tell youself "IF I do this, then I can try that".
On the other hand if you have the thought process of "I'm going to play this shot, stop here, take that shot, stop there", the moment the plan is screwed up you loose confidence and have to re-evaluate. Many people say to themselves "I HAVE to make this shot and stop EXACTLY there". They make a little mistake and they're mentally out of it. They get frustrated and at that point the battle is already lost.
Planning is good. Just don't let yourself get too attached to that plan because under presure, you're more likely to get out of shape. Take each shot at a time and don't get ahead of yourself.
Don't just plan where to put the cueball, plan where the cueball will be in relation to the object ball should you miss your shot. Play as many shots to nothing as you can.
I learned alot from Jimmy Reid's videos, wish him all the best.