It's almost a commonplace to say that neither the trials nor joys of our particular vocation are what we expected. I'm starting to wonder whether the Holy Spirit doesn't want us to indulge our vainglorious ideas about what a vocation will be like (e.g. marriage, religious life, priesthood) in order to get us to go ahead and do it. Once we have followed the Spirit in, then the truth can be revealed.
Anyway, there have been a lot of unexpected discoveries about priesthood. Celebrating the Eucharist as a priest has shifted my whole sense of it. For example, I always looked at the sacrifice as something that happened to the bread and wine. But now I look at it less as something that happens to the bread and wine, and more as something that happens to God; something God goes through as it were, his passio.
On a practical level, there are aspects of the Mass that I honestly never knew about before being a priest. Now I probably went to Mass 2,500 times or so as a layman, but there were prayers I had never heard and things I never knew about. Here are three examples:
1. The secret prayers. I had seen priests praying sotto voce, as it were. I had even heard lots of priests say the secret prayers in a regular voice. But a couple of them I just never knew about, like the prayer during the purifications, which has become one of my favorites: Lord, may I receive these gifts in purity of heart. May they bring me healing and strength, now and forever.
2. The Kyrie after the Confiteor. If option A is used for the penitential rite at the beginning of Mass, that is, the I confess to Almighty God..., the priest gives the absolution after the Confiteor, May Almighty God have mercy on us... and then is supposed to say or intone the simple Lord have mercy... before going on to the Gloria or the Opening Prayer, as appropriate. Before being ordained I had literally never heard any priest do this. When the I confess... was used, they would give the absolution and then go straight to the Gloria or Opening Prayer, omitting the Kyrie. I've asked a few priests about this. Most said that I was wrong, but others admit that they are supposed to do it but don't want to. The rubrics are pretty clear, though. I say, why omit this beautiful piece of our tradition, and a great opportunity to insert a little Greek into the Mass?
3. The prayer after the Sprinkling Rite. Here's another one I had literally never, ever heard before being ordained myself. If the rite of sprinkling with holy water is used in place of the penitential rite, the priest is supposed to pray this prayer over the people when it is done: May almighty God cleanse us of our sins, and through the eucharist we celebrate make us worthy to sit at his table in his heavenly kingdom. Again, I was shocked to see this prayer in the Sacramentary, because I had never heard it at Mass. My experience was that the Sprinkling Rite just ended and the Gloria would begin. Or, as is widespread practice among the brothers of my Order, the Gloria is sung during the sprinkling. This, of course, necessitates the omission of the prayer, because when you're done sprinkling it's time for the Collect. Age quod agis, I say. Let's do one thing at a time. Plus I like the prayer; it's been one of my discoveries.
As one of my confreres, a lay friar, likes to say, "Making up your own liturgy is the height of clericalism."