Thanks everyone for the helpful responses. That Ronnie James Dio reference was a great example. And thank you Adolph for sharing your story with me.
I should note that I do have problems with my cervical spine. I was born with several fusions between some of my vertebrae, including a partial fusion between my top vertebra and my skull. I wonder if this limits my ability as a singer, although the doctor I see regularly for treatments for neck pain assures me that physiologically those issues shouldn't be impacting my singing (he understands the human body and its function better than probably 99.9% of the doctors in the world, although he is not a trained singer).
I'm currently taking a singing class for the first time. I'm 28 years old. I've wanted to be a singer my whole life and always assumed I wasn't born with the chops to really be the kind of singer I idolize. I still suspect this is the case. In all my years of attempting to sing (15+), I haven't been able to improve my range at all on my own. In two months of classes and practicing proper breath technique and opening of the throat and posture and all of that, I feel like I've slightly improved my technique as a singer, but my range doesn't feel like it's going anywhere. Sure, I can support the top of my range better now with proper technique, but it's still the top of my range, and I can't sustain it for very long. Some days I feel like despite learning some technique, my singing voice is still fundamentally the same as it's always been.
Can anyone explain physiologically what creates the limits of one's range?