I think having a consistent pre-shot routine is the first step to overcoming mental weakness at the table. Every ball including the duck sitting right over the pocket should be shot with the same routine. Here's a typical pre shot routine:

1) Survey the table and decide what the right shot is. This includes what ball you are going to hit, what position you are going to leave you cue ball after the shot, and the best way to get that position. Before you step to the shot you should already have decided speed, stroke, and where on the "clockface" of the cueball you are going to be hitting. By the time you are in your stance you should have already imagined exactly what's going to happen on the shot. I mean actually seen it in your mind.

2) Walk into the shot. What I mean by this is that now that you have decided the shot and its direction, and imagined it happening, with your body facing the shot direction, walk forward directly in line with the direction you will be shooting and get down into your stance. If you realize you are not aligned properly, stand up and approach again.

3) The next time you are playing seriously but you feel relaxed, count the number of warm up strokes you take on shot and the speed of the warm up strokes. Whatever the amount of strokes and the speed, you should be doing the same or similar for every shot (three warm up strokes is a typical minimum). Sometimes it is advisable to take a few more strokes for difficult shots. But it is almost never advisable to take less strokes. There are two situations where many players decrease their number of strokes, and often speed up what strokes they do take: when they have a hard shot that they are not comfortable with and when they have an "easy shot." People rush the hard ones because you want to get the shot over with; people take less time during easy ones because they are [seemingly] easy.

Shots you are uncomfortable with are the most important shots to not rush, yet many people do the opposite. As for "easy shots," there are no such things; only less difficult shots. If you are like many people, this might sound familiar. You devote a lot of concentration to a difficult shot and make it. The next shot is a duck, so you barely focus on it and you miss it!

4) Two of the most important thing on any shot are to stay down and to follow through. This may become unnecessary eventually (it hasn't for me) but I try to make it a point on every shot of saying to myself "stay down and follow through." It's like a relaxation mantra, except is self-instructional.

If you get used to a consistent pre-shot routine, not only will the steps help keep you from being distracted by the "grumpy, unhappy opponent" trying to throw you off balance, but it will be a mechanical fall back position for when your mind is wandering. Even if your mind is wandering, as you say, if you go through the motions of doing something because it is habit, your instincts will be much more likely to take over than if you didn't have this routine to fall back on.

Remember before I talked about counting your strokes when you are focused but relaxed? That's because when we are relaxed (and confident) we often play our best pool. Relaxing at the pool table is difficult. You're facing a long distance cut shot that decides the match--how can you be anything but tense? Well there are some ways to "force yourself" to relax.

There have been studies done which show that if a person's body is forced into particular states, the mind will follow. Example: You are on the ground floor of a building in which the whole floor trembles imperceptibly because of the machinery in the basement. Even though you don't realize the building is shaking, you may feel tense and possible scared because your mind reacts to the trembling imparted by this outside source with feelings that, paradoxically, usually cause us to tremble, i.e., tension and fear.

One of the best ways to tell if you are tense is to check whether your jaw is tightly clenched. This is a sure sign of tension. The next time you feel tense at the table, if you remember, check whether you jaw is clenched. If it is, consciously relax it. Your mind will follow hopefully, and you will feel less tense.

As you probably know, it is important to have your back hand relaxed when you are shooting. If you have a death grip on the cue you will probably miss. Not only will you improve your game by being aware and making sure your back hand is relaxed, but doing so will also feedback to your mind, making you more mentally relaxed.

Finally, if you want to explore the mental aspect of the game in depth, I recommend: The Pleasures of Small Motions: Mastering the Mental Game of Pocket Billiards by Robert T. Fancher. I think this is the best book out there on the mental game.