Golf is not a game of perfect; it's a game of a thousand variables. First, you--the imperfect human--is the greatest of all variables. One day you may play great and expect to go out the next day and do the same. Wrong. Golf don't play that. You may think you're doing everything exactly the same, but because the tolerances (+/- standard deviation) in golf are so narrow, it doesn't take much to get the ball hooking, slicing, topping, fatting, shanking, etc. Remember, even the biggest club face is only a few inches across, so that means you, at most, have a few inches of lateral and/or vertical leeway; that's not much room for error.
The other big group of variables include Mother Nature--the golf course conditions. First, is it an unfamiliar course? How difficult is the course design? How far back are you playing? What's the wind like? The temperature? The humidity? Rain? Fog? Terrain? Your lie? The grass length? Distracting shadows? Bright sun? I could go on and on.
Then there's simply probability and chance. Yes, I'm talking about "luck." You can hit a good shot but get a bad bounce or kick. You can hit a tree and have it bounce back into the middle of a fairway, or if you're unlucky it can bounce the other way and end up further in the woods. You might roll into a bunker and end up with a bad lie. Your ball can simply be in a bad position on the fairway, making your next shot tough. No matter how good you are, there are aspects of the outcome that are difficult to control. Chance is in charge here.
So, getting back to you--the group of variables you have the most control over. It's important to be able to analyze what's happening and make reasonable adjustments on the fly, or your day of misery on the course will continue. Sometimes one small adjustment you make will make all the difference. Some days, nothing will work. Happens to the pros too, so don't sweat it.
As so many wise sages of golf have said, what's the ball doing? It really will give you clues about all of the variables and how they're affecting your game that day. For example, if I am suddenly hitting straight pulls low and left, the first thing I can do is run through a fundamentals checklist in my mind, and I should start with the club face--probably too closed at impact relative to the path--and go backward from there. In this case, I may have unconsciously taken too strong of a grip (turn the grip leftward until I see the ball straighten or even start right a little). Maybe my grip pressure is too firm, or I'm gripping harder during the swing. Maybe my stance is too open or too closed. Maybe I'm too tense on the downswing and trying to muscle it. These are all things that may help correct recurring faults.
There is no magic elixir, no sudden epiphany, no all-encompassing panacea that will suddenly turn you from high to low-handicapper on the course. The swing thought or tip that works today may not be the one that works tomorrow. But there does seem to be one universal truth: A return to fundamentals can reverse your fortunes.
So start from the beginning and progress through the checklist. If you're a diligent student of the game, you likely know a lot about your particular tendencies and errors, your strengths and weaknesses (this blog is mostly about my own particular quirks, strengths, and weaknesses).
Begin with alignment, setup, and posture. Next the grip. Remember GASP (grip, aim/alignment, setup/stance, and posture) as an overall starting point. Next overall tension, rhythm, and tempo. Think of each one of those areas; there are so many tidbits of information associated with the fundamentals of each one. That's why golf is so hard; it's a game of managing mistakes, because the mistakes are coming--even when you're playing great. The golfer who makes the fewest mistakes (or the less serious mistakes) is going to score well.
Here's what you should NOT do. Don't rush. Don't have 15 different swing thoughts. If a zoned, blank-minded swing isn't working that day (and unless a flaw becomes suddenly obvious), you might be better off to only focus on two fundamental areas as potential quick fixes: GASP and keeping the head back on the downswing. It's likely that going any further with backswing, downswing, and impact swing thoughts and modifications will only increase your misery on the course. Save that stuff for practice.
Rushing is something I can occasionally do. I'll stop running through my usual preshot routine and stop taking practice strokes. I'll stop thinking about GASP. This is tension creeping in. The best thing to do is RELAX EVEN MORE during these bad moments and remember GASP (what happens before the swing) and for the swing, just keep the head back.
And don't try to make a miracle out of a bad situation, especially when you have no business (i.e., no such talent) doing so. I will soon publish a Trouble Series on how to deal with trouble lies (and they're coming). If you get over the ball and you don't feel right, back off! Go through your checklist on how to deal with the trouble and what you're trying to do.