St Thomas Aquinas says there's essence and existence: two aspects of your being. You are a certain kind of thing (your essence) and you are that kind of thing actually are-ing, in this moment, right now (your existence). The essence is like a description, a form, a shape, a blueprint, and the existence is the fact that something described by that blueprint has just taken a bite of pie.
So, parallel to this, in the realm of acting and morality and choice, you can fail to rightly act in two ways, each corresponding to one of each of these aspects to being. First, you can fail essentially. This is to say, you're so screwed up inside that you think the wrong thing is the right thing, or else, again, you just don't have the strength to do the right thing. No virtue, no judgment: you're just a dumbass. Something is, in a limited sense, essentially wrong with you. (Don't take that to be a judgment on your whole being. It's not. You're a beautiful child of God.) It's your misshapen imagination or your weak biceps that are betraying your attempt to do the good. But then there's the more mysterious aspect of failure. You can fail not because you think the wrong thing is the right thing or you tried to do the right thing and didn't have the strength, but because you just ... fail to do the right thing. Existing is an act. It's possible to fail to do that act. Acting is also an act, and it's possible to fail to do that, too.
Your will to the good is like a rock-climber halfway up a wall. He or she has to be actually holding on, doing something, the entire time. But it is possible for him or her to just ... fail to hold on. And if they do that, they're going to fall and break stuff. Maybe all the stuff. That's the mysterious existential failure at the heart of a lot of our bad-acting: it's possible not to act.
Now, complete and total existential failure is unusual (outside of a Star Wars film). It's rare that someone just loses completely the will to live. You rarely fall all the way down the cliff. Usually, you fall a bit and get caught on some snag or ledge sticking out. Which is to say, you've always got some lower-order functions that are pretty good at doing what they do, and when you fall into failure, they take over. Depressed? Given up? Don't worry. You won't at first lose the will to eat. (At least not in my own experience.) That lower-order process will keep you alive and provided with at least a menial satisfaction even when you are tempted to stop trying at every other area of your life.
Of course, just like eating when you're depressed is, ultimately, not a good thing (so says my doctor), so a lot of the actions of those lower-order functions aren't that good when they take over. They're good in their place. Everyone has to have food, and hot dogs and ice cream are, in a limited sense, foods. And the satisfaction you get from eating is a good thing. But asking ice-cream to supply me with the satisfaction that should normally be supplied by rocking my vocation is asking for trouble. It will take a lot of ice cream to make me feel as good as watching some student succeed at doing something hard that they couldn't do before my class.
So when we give up on doing the really good thing, what we're usually doing is not falling all the way down the cliff and dying on the rocks below, but rather falling back on something we're good at that isn't actually the thing we're supposed to be doing in that moment. The soldier cowering in a foxhole when he's supposed to be going over the top and attacking the enemy isn't choosing something bad in itself. Keeping your head down when high-velocity projectiles are flying around is generally the right thing to do. But it isn't what the situation here-and-now calls for.
What we often call "choosing the bad" or "making the wrong choice" isn't about actively choosing the "bad". It's more that the situation calls for a really hard good, and we are about to do it, and ... then we fail and fall back into doing something that we're good at that will provide at least some kind of satisfaction. Weirdly, it's about the will to be. Are you willing to choose to be what you are being called to be right now? Or are you going to fall back into doing something else that's easy?
That's the existential question of morality. The choice to act well is the choice to be; more than that, it's the choice to be awesome, because that's what humans have been called to be. You have the opportunity to make that choice. Choose to be awesome. It will be hard, but it will be worth it.