The loss of one's first fervor can be very hard. There can be a lot of confusion and interior vertigo, what John of the Cross calls the 'dark nights.' A healthy grieving and letting go are necessary. Nor is it a one-time thing; someone may think that he has passed out of his first fervor only to realize later that he hadn't really dealt with the loss at some deeper level. There's a sense in which a spiritual person is always passing out of first fervor, in the same way that an adult always remains the child he was in his family of origin.
But to manage all of this in a healthy way is critical to spiritual growth. If we have not let go or grieved well, we can continue to indulge the part of ourselves that pines away for the clarity, energy, and excitement of our time of first fervor, distracting us from the grace and work of the current moment in our journey.
For whatever reason, the romantic metaphor works for me. My first fervor in my conversion and my religious life was like a fun and energetic girlfriend. She took me to new places and introduced me to new experiences. She was sweet and attractive. But, as time went on, troubles started to appear in the little spiritual playground of our relationship. I began to see how she was possessive and exclusive; our relationship was a spirituality that didn't lend itself to begin shared with others. She had no interest in children; ours was a relationship just for us, a spirituality just for me to enjoy. More than anything, she couldn't seem to handle prayer when it began to cease having a lot of feeling or affective reinforcement. As prayer began to turn into the 'uninteresting wilderness' (as Thomas Merton called it) she started to break up with me for good.
I was a little lost at first, but over time I worked through it. At this point, coming up on nineteen years baptized, having spent the majority of that time in religious life, I look on my first fervor with fondness, knowing that it was the right relationship at the time, but without any longing to go back.
Not to be on a Mr. T. Experience kick, but this song captures a little of my attitude toward my first fervor in religious life: