So I'll tell you, and where I'm wrong, maybe other teachers can chip in.
Imagine a job where the same sales pitch begins year after year. You make it. You aren't even entirely sure that the potential customers listening to your pitch are in the market for what you are selling. To be sure, they are a captive audience, but that guarantees absolutely nothing. And they are going to have to gauge their own self-interest in the enterprise over and over for a very long time. Nearly a year. The product does not come pre-assembled. The customers are going to have to put it together themselves. To have any hope of closing the deal, making the sale, satisfying the customer, you are going to have to offer a lot of incentives, maybe even some "freebies" just to keep them on the line.
Or a play that has so many acts that it takes most of the year to perform. The play does not fit any distinct pattern of drama or tragedy or comedy. That is decided by the actors as they go along, and individually, not as a group. For some, they will triumph from the chaos. For others, they will be their own worst enemies. The outcome is uncertain, and when the next performance begins a year later, all of the actors in the play are different. Except you.
You will be playing the same role each time, but your performance will be informed by your previous performances, and unlike other plays, your character ages each time he or she takes a bow. The next show is the next year.
Or a game, maybe a carnival game, where your job is to give out the prizes. There are no particular rules, or at least not the same exact rules for everyone who plays. There may be one who throws perfect darts and pops the balloon time after time, but he or she will have to throw those perfect darts day after day for months, because one slip up and someone else may get to choose the biggest stuffed animal. Some may never hit a single balloon, but there is a good chance that they, too, will come away with some kind of prize. Maybe not the kind a boy gives to his girlfriend. Not everyone can pin the tail on the donkey.
And you? Whether salesman or actor or carney, you must maintain your enthusiasm for the endeavor.
But that isn't really it; that's not the challenge. Because it isn't hard to generate the enthusiasm. You the salesman believe in this sale. You the actor believe you were born to play this role. You the game master would likely rather give out the prizes than not, so you pass out the encouragement and share the tricks of one who has won one of the big prizes him or herself.
No, ultimately those metaphors aren't the ones that you are looking for. How you really see yourself is as a magician who is going to stand in front of an audience. It is an audience that has seen a lot of tricks, maybe even some of yours, and they know that the show is going to be long, that they are going to get restless in their seats, and that they fully expect to be amazed or, yes, they will want a refund, and if they don't get it, they will make sure the next audience coming in knows the weaknesses of your show.
Between shows, maybe you either try to refine your tricks or look for some new ones. But there is more than practice involved. After all, over time, the cards are going to get worn, the props faded, the magician's coat frayed and tattered. You are going to need more than practice; you are going to need a spark. The problem is that when you've had it, you never quite knew where it came from. And so each year, year after year after year, you walk into that classroom and hope or pray or will that the magic will be there.