How many times have you stepped onto the first tee and your goal was to shoot the best score that you have ever shot?
Chances are yes and chances are you failed. The goal wasn't a reality. The completion of your goal, simply put, should leave you in a position to shoot your best score.
Here is how I think one should approach shooting their best score:
You can only have ONE best score, so this should never be your goal because you are most likely going to fail. What is an attainable goal is trying to go out there and not short side yourself all round or put a new shot in play that you have been practicing. Work on consistently shooting 2-4 shots above your lowest score. Once you can do that, you will be able to have a better than average round and potentially shoot your new lowest score. It is more about "how consistent can I get and stay", than it is "I want to shoot this number".
From here what can we do?
The first and most important thing that you can do to better manage your expectations is to become more aware of what you are expecting.
Before your next round, ask yourself these questions:
What am I expecting to accomplish from this round?
What am I expecting to be a challenge?
What aspect of my game do I expect to lean on the most?
These questions will be followed up with:
Where did these expectations come from and are they realistic?
If yes, what steps need to be completed in order to succeed?
The trouble with expectations in golf is the infinite amount of uncontrollable variables. You could go out there and expect to not miss a fairway (exaggeration, yes, but you get the point) and then 5 holes into the round, catch a wind gust that throws your ball slightly into the rough and now that expectation has been failed.. Whereas if the expectation was to not get yourself into any trouble off of the tee, you could simply miss a fairway in the right spot and still successfully keep that expectation intact.
Only when our expectation is to focus on one shot at a time and we are giving each shot the respect that it deserves, that we can allow ourselves to get out of our own way and play our best golf on a consistent basis.