I notice that you didn’t mention theme among the basic elements you had in place. You didn’t say much with your question, so forgive me if I wander off into speculation. In fact, I’ll toss a couple of ideas out there and hopefully something will stick.
Several writer friends of mine and certainly many successful pros are plot-focused, genre writers. There’s nothing wrong that. My genre writing buddies rock at plot; it just pours out of them, but I just can’t do that. I’m not pumping myself up. I’m saying I literally am incapable of creating any plot without knowing what I’m trying to say as a writer in the story.
There are just too many possibilities. Consider how many car chases there could conceivably be for example. How do you know what kind of car chase works for your story? Knowing the point my story intends to demonstrate, helps me understand the job of each scene or chapter. Combining this with the need to develop the character really helps hem me in, reducing the possibilities to a manageable few. Some people are more creative without constraints, but I need them.
Another possibility I’ll throw out there (and I really am speculating here) is that you’re writing the wrong story. You didn’t say much, but you mentioned you had the backstory set, but only had a good idea of the story. This suggests you understand the protagonist’s past better that their present. But your story occurs in the present. Consider this.
I’ve noticed in the work of my writer friends a funny tendency to hide from their own character’s emotions. This takes the form of a tragic and defining backstory contrasting a more standard, factual, plot-based story. Often they reference the backstory as exposition when the story’s current events run out of steam. If you find that your protagonist’s backstory excited you into writing in the first place and not the, “basic elements of most fantasy stories,” it might suggest that the better story is the backstory. If so, write that story. Just a thought.