There are many people who enter seminary or religious life, test it out for a period, maybe even make it to the brink of ordination… then realize that it is not for them, afterall.
Was a mistake made? I don’t think so. Rather, a following of God’s Will to the point that it led them occurred. What happens next? Well, that also is part of the journey. Sometimes one who walks away from such a vocation knows very clearly where s/he is headed and why. Other times, there is a period of insecurity or wandering. This, too, is discernment.
Now, that isn’t to say that one doesn’t need to persevere in prayer and dedication. As Fr. Joe rightly noted, more than a few priests have left ministry perhaps because they just couldn’t handle it’s rigors and got burnt out or rusted out; because they got drawn away by some worldly concern, a woman; because they let their prayer life wither and didn’t allow God to be in charge. Others entered and somehow got ordained for, perhaps, the wrong reasons or with a faulty understanding of what could/would be.
So, it happens. But, yes, it’s a lengthy journey. And you have checkpoints to help you understand if this is the road to continue down or not. There will be highs and lows in any way of life, and this all is no different. All one can do is take it a step at a time, and see where God leads.