That is an absolutely crucial and huge question - with, what I expect, is a uniquely personal answer. But I have prayed about how to answer it and share these thoughts which have worked best for me:
The first thing to remember is that it is our habitual response - usually supported by our culture, our pride, and even our own bodies - to try to master fear. For the most part, the attempt to "master fear" is to repress or to deny it, leaving its source undiscovered and, therefore, unhealed. So the "mastery of fear" is futile and (according to the Course) counterproductive to the Atonement. The proper course for healing, ACIM says, is not through the "mastery of fear" (which is impossible) but through the mastery of love, instead. But what exactly does that mean?
All fear is premised on ONE belief: that I am alone and on my own. In other words, separate from God, therefore, unlike the way that He created me. And so any thought to "master fear" is to affirm not only that there is something real to fear but that something not of God has the power to harm something that He created - that there is a force more powerful than God; a will opposed to - and able to - dominate His Will. Absurd.
To me, to "master love" is to train myself into the habit of consciously reaffirming my choice to be connected to Love (my favorite prayer is: "I let Love decide."), no matter what appears to be before me - One fear at a time. "Big" or "small" doesn't matter. And from that one simple different perspective than what the ego's world has taught me, I can look with fearless clarity on the source of fear to let it be dissolved in Light.
For me, it's been a process of "baby steps" toward Love - of testing my connection to the power that created "the universe of universes", and then just judging by the results (which are far more convincing than any words); A gradual - and organic - path of learning that I am safe, no matter what seems before me (Or within me, for that matter.)
Or, to paraphrase the introduction of The Rules for Decision: "Decisions [of whether or not to go into fear] are continuous. You do not always know when you are making them. But, with a little practice in the ones you recognize, a set begins to form which sees you through the rest. It is not wise to let yourself become preoccupied with every step you take. The proper set, adopted consciously each time you [FEAR], will put you well ahead."
This is "the purification process" - the testing of what ACIM says - its truly "practical application" - One fear at a time. Until the last temptation to believe that anything not of God could hurt me - and nothing of God ever would - must simply fade away as the tiny, mad idea that is its premise becomes increasingly crystal clear and then is eventually rejected altogether.
The results prove that you and I are safe. So safe that eventually, with your and my experience of that safety through our conscious (intellectual) reconnectimg to Love, you and I can get it emotionally that there is nothing at all to fear and then we're almost home.
But in the meanwhile, though, we must remember that it is unwise to "become preoccupied with every step" for that will lead us back to guilt and fear. So please be gentle with yourself along the journey. Each tiny choice for Love is progress! For not even the smallest effort is wasted, ever.
Just trust in that. And, if you have the time, please read Lesson 109, "I rest in God." It has always been incredibly helpful to me in my own personal times of crisis.
I pray this has been helpful to you. Your progress, dear, is also mine!
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* Carmen Cameron, who is a founding member of Course in Miracles Society, has been teaching classes in A Course in Miracles since 1994. She was a presenter at the 2009 Miracles Conference in San Francisco and is scheduled to present again at the 2011 Conference. Carmen's website is: http://peaceful-path.blogspot.com/