This thread asks some good questions about what then mental game really is. Nerves over the ball to me seems just part of the actual technique/routine of playing of the shot. If I've had a good warm up and just hit 5 solid 3 woods then stepping up to the first tee is not really a particular mental challenge. We clear our minds of all unhelpful thoughts, maybe keep the one that helps us most and swing away.
I sometimes struggle with a short game shot if conditions are unusual or its a tricky one. If you flub one or two it feels like a 'mental' issue (bounceback!) but for me it's just essentially just doubts over the technique I'm about to use (lack of warm up or practice is usually the cause).
If the mental game is anything distinct I see it as the ability to remove from your mindset the external circumstances of your round and crucially how well you played the last few shots. These are the things that inhibit your usual technique and shot playing if you let them.