The "first fervor" describes the state of intense energy and openness that characterizes the beginning our spiritual life. It may be the zeal of a convert or "revert" or the grace given to someone as they begin a new vocation.
First fervors are wonderful times, replete with grace and spiritual energy. People who are in them are inspiring and encouraging. The problem with the first fervor, however, is that it ends. Mine lasted for about two years from my sacraments of initiation and then fizzled out, hard.
The loss of the first fervor can be an intense spiritual challenge. Sadly, I don't think some ever get over it. Suddenly we don't have the taste for prayer and devotion (on the natural level) to which we had become accustomed. The novelty of the experience of the interior life wears off, not to mention the novelty of the material elements of religion. We find ourselves falling into old habits and sins that we thought we had overcome, no longer finding what we thought was the fortitude and spiritual energy to fight temptations.
The disciple must accept this transition as a blessing within the loss. The grace of God is inviting the soul into a faith that is more intimate and pure precisely because it has a weaker foundation on the natural level. If we only grieve for the imagined loss of the consolations and the intensity of feeling we had in our first fervor, we miss out on grace and don't manage to grow up spiritually.
Longings for the pleasing feelings and easy internal acts of piety of the first fervor must be fought as a temptation. First, they betray a certain materialism in the spiritual life which is seeking after commodities to be enjoyed rather than the naked will of God. Second, these temptations suggest to us that God has not given us what we need, and that the grace of faith we have been given is somehow insufficient. We are greedy for certainty and religious feelings. If we start to base our so-called spiritual life on this, we are lost. Luckily, the sensuality to which we have committed ourselves will betray us in the rest of our life and if we are humble enough we will notice.