The most common narrative conflict we see in fiction today is what is know as Person Vs. Person. You have your protagonist working with/against your antagonist and the two play off one another creating the narrative conflict throughout the arc. This generally ends with one or more deaths/incarcerations what have you.
But there are other forms of narrative conflict that don't involve an antagonist.
There's the old standby Person Vs. Nature, which is generally exemplified by books like Hatchet and short stories like To Start a Fire (which I read in Middle-School and still haunts me to this day). For those of you not familiar with the term. It's generally a protagonist thrown up against the elements. So a blizzard or a scorching desert. It's a person vs. the jaguar stalking them through the night. (Though there are personified examples of person vs. animal where the animal definitely feels like a definitive antagonist).
We also have Person Vs. Self, Person Vs. Society, and others. They important thing is that it's a protagonist thrown up against something/someone. So, to simplify this:
Conflict = the Protagonist VS. Something/Someone
That's it. That's conflict. Now, the truth is, not all fiction utilizes the antagonist as a person or a beast, or monster. Some fiction works quite well without an antagonist. It utilizes some other sort of conflict to get the job done.
I tend to steer away from centralized antagonists in my work. Not consciously, mind you. While my stories do tend to have a villain, that character may not actually be the antagonist of the story or they may not have much of a presence within the story. There's nothing precisely wrong with that. There are going to be misdirects, on occasion, when it comes to identifying the root of your story's conflict.
A trilogy I was working on last year had elements of different narrative conflicts with no unifying antagonist, and I was having some trouble sorting out why, as I'd plotted the damn things. It only later occurred to me while writing book three, that the antagonist had been a steady presence all along. I just hadn't put two and two together. Now, that said, that still wasn't the driving conflict.
The best example of another author who does what I call an "ephemeral antagonist" is JK Rowling with Voldemort. He is a felt presence throughout the series. His agents are everywhere, his name on the lips of those still around convinced he's gone for good. He's a cloud over the proceedings. No one can deny he's the series antagonist, but conflict isn't always driven by his hand directly. It's generally a consequence of his presence, but not necessarily because of direct actions he may have taken.
I generally feel that antagonists can be the "Man behind the curtain". Pulling strings, always present, but on screen for a short amount of time while still maintaining maximum impact. Because of this, conflict tends to feel more self-driven on the part of the protagonist. You can get away with false leads and even pseudo-antagonists that mask your main villain's purpose. It's not just about conflict driving the story at this point.
We talk about plot driven stories VS. character driven stories frequently, and we should. But you also need to ask yourself, what kind of conflict is driving this story? The underlying thread should remain the same through the story and it should boil down to a single concept.
If I boiled down that trilogy I talked about earlier, it'd be pretty simple. The protagonist really just wants her father to be proud of her. That's her overriding desire, the conflict is that he's dead and that's never going to happen so she keeps striving higher and higher. The other conflicts are secondary to that goal. So the conflict at the center of it, is not with the antagonist really, but with Self. This is a common thing to see within YA narratives really, as they tend to be about Growth regardless of genre. So while you may have these characters going through all of these things, their core conflict is going to be Self and on occasion Society with villains thrown in to give you something to focus rage on.
Figure out what your core conflict is, that thing driving you character forward, and then build your narrative arc from that. This is why many people will tell you to start your story near or at your inciting incident, as this is what the conflict comes from. It's going to inform everything else that happens, and will be reflected in the ending of the book. If you can do that, your resolution is going to feel a lot stronger, more earned.
Don't be afraid of not having an personified antagonist in your story. Not every story is going to have a mustache-twirling villain to focus on. Sometimes, it's just the wind howling outside the window. It's the crushing weight of an immovable force. Conflict is core to narrative. So if you're struggling with your story, go back to the conflict.