Going to the Next Level
You already have the answer to reach the next level, but it will require effort and practice to put it to use. So stop looking for another answer. The mental golf game is simple to understand. It's the doing that is the hard part, but like everything else it gets easier the more you practice.
The secret to success is to understand and know what works. Then, put all your energy and focus on doing the work to improve until you are successful. Unfortunately, most of us do not want to do the work. Instead, we keep looking for another book or short cut to success rather than doing the necessary work required to improve. Think of it this way - first you work, and then you get paid. Have you ever found it to be different?
When you can play one shot at a time totally focused on your target with only positive expectations of making your shot if all goes as expected, you will be at the next level. Yes, it is a lot easier said then done. But, how do you expect to reach that level without doing the work to train your mind to think that way?
Just like it takes proper training and lots of practice to develop the correct muscle memory to hit, chip or putt at a high level of proficiency without thinking, the same holds true for your mental game. You don't have to reinvent the wheel every year. Just keep polishing your wheel until you get it as close to perfection as you can or desire.
The most difficult thing to train you mind to do is to continually focus on positive goals and processes regardless of your circumstances, especially if they appear negative to you. Remember - Your present circumstances and handicap are the result of your past thoughts and actions. Your future circumstances and handicap will be the result of your present thoughts and actions. So choose all of your thoughts and actions wisely. Be patient, do the work required to improve and enjoy your journey to the next level.
P.S.: Did you know that there are different types of eye sight and eye coordination capabilities? When playing a round of golf over 5 or more kilometres, and having to aim every shot or stroke, seeing properly is a key element for better golf. Here is a book I have personally read and can recommend: Improve your eye coordination capabilities