One way of zeroing in on what that one most important thing is to make a short list first. Then, imagine life without each one. It's a way of backing in to feeling what you value most highly, if you don't know what it is for you.
This is partly a personal post-because I had been thinking about this for myself. What is my most precious priority for me? And, how will I protect it? Then, as will sometimes serendipitously happen in therapy practices, a patient came in talking about that very subject.
In his case, it was quite sad as he had lost the most precious thing to him---so, here was a case of recognizing what was personally important, after the fact.
Whatever it is, it will be known only to you. Others who are close to you may be able to make a good guess, but, only you can truly declare what that essential-to-you thing is.
It may be your religion, it may be your marriage, it may be your health, it may be your freedom, it may be your family's well-being, it may be your work, your business, an interest you pursue with a passion, maybe it's a person; it could be so many things.
What is it for you?!
What I am considering myself is, how can each of us, once having identified that thing, sustain and protect it. It seems that there must be a commitment.
I remember one former patient who had suffered great personal losses in her life. She once told me that, at a certain point, she decided that she wanted a life that contained as few stressors as possible. To that end, she had made many choices to create that condition for herself.
Sometimes we may have to give up something else to keep our one thing secure. We may have to do things pro-actively to nourish it. Above all, try to not have happen what did to the fellow I first mentioned who, inadvertently, despoiled his own most important thing---by neglect.
He, sadly, did not realize the significance of this thing in his life until it was lost.
So, first and foremost, identify it for yourself!