The question posed here is complicated, and I thought about it before, too, from time to time. So I'll try to share my experience (mostly using the example on the calculus), conclusions and thoughts. But please remember that they may be quite subjective.
TL;DR: I admit, that brains of different people are working in not the same way. Rate of learning of different things is different for different people. But I still believe that the person with more-or-less healthy brain can learn anything, if they are investing enough efforts into it.
I want to start my story with some noticeable observation: when people talk about the talent in some field, they tend to overgeneralize things. For example, I've heard about "talent in mathematics" many times. But what does it ever mean?
I myself had almost no problems with number theory, analytic geometry, discrete mathematic and logic in the university. But I had very huge problems with understanding calculus and partial equations. Those all are the different branches of math, and you can see here, that the same person can be "talented" in one of them and "not talented" in another.
Here is more detailed story about how I dealt with it if anyone is interested:
When I started to learn calculus, it just... didn't fit into my brain. My intuition and imagination worked good in geometry, but they were helpless here. Every time, when I was reading the proof of the new theorem from calculus, I strained myself hard to understand every single line, every logical conclusion. But every time, when I had reached the end of the proof, I forgot the beginning of the proof! So the general meaning of proof slipped away from me. It was painful. 
Professors wondered, why I'm able to solve complicated geometric, logic and number theory problems, but don't understand anything from calculus. Calculus professor tended to think that I have no talent in math and should try another department. Number theory professor argued with him and said him that I'm talented and that he should give me more chances to pass calculus (this professor was so kind for me and helped me so much... I will never forget it
)
I've understood that calculus is important for better understanding my favorite math branches, and for furhter learning of more complicated fields, too. So since I liked math and wanted to learn it deeply, I should learn calculus, even if it is hard.
I have spent two last months of the summer between the first and second years of the University, learning calculus to pass the exam on it (I've failed it several times at the June session). As I said, every time when I reached the end of the new proof for the first time, I forgot the beginning. But when I kept to re-read proof again and again, thinking very attentively about every logical step, at some point it fitted into brain, and stopped to slip away. So, usually at the third or fifth time I usually started to understand what's going on in it. And every time when I was able to extract and understand the idea of the proof, I wasn't need to remember the whole proof anymore. I remembered the idea and was able to reconstruct the details. I repeated this hard work again and again with every calculus proof.
At the end of the summer I've noticed, that I started to understand new proofs from calculus more quickly and to extract the main ideas from them more easily. Moreover, I've learned finally how to use my geomertic intuition and imagination to understand those theorems. I learned how to draw some objects from calculus in the way, which is helpful for understanding them. So I illustrated theorems and proofs with a pictures, every time when it was possible and had a meaning. It helped me additionally. Finally I had drawn a big scheme with the connections between different definitions and theorems, and it helped to understand a bigger picture. By the way, my father was helpful, too. He had only very basic knowledge in calculus, so he was able to explain me anything only on the early stages. But I've still continued to explain him the proofs every evening after learning them, so we discussed them until we both had catched at least the main idea. It helped me a lot as well to understand them and fit them into my head better.
As you have probably guessed, I've passed the exam on it at Autumn.
Moreover, I even started to understand new math fields better and learn them more quickly after doing that hard, but useful work.
What conclusions did I done from this experience:
- There is no thing like "talent in math". Sometimes the brain can perform some particular types of thinking and learn to manipulate some particular types of objects easier, than another. But math is very big, so it require different way of thinking in different fields. You can use those ways of thinking which are easier for you first, and then develop those which are harder.
Now, how this result is appliable to the art... I think, that "talent in art" is overgeneralization, too.
I've noticed, that when I draw the comics, there also some things which are harder for me, and some things which are easier. For example, I like to draw very neat lines and make coloring. It is more-or-less easy for me to draw weapons. But it is hard for me to draw motion and people's emotional expressions on their faces. Also I don't understand which things I should draw more attentively, and which can be done less neatly. So I'm drawing everything neatly just in case.
But anyway, I believe, that those hard things will become easier with experience and attentive learning and thinking how to do them, since this worked with calculus... which take us to the next point:
- Brain is very flexible, you can teach it many things if you will try hard and spend enough time. Again: you can try to lean on things which are easier for you first, and then learn hard ones step-by-step.
- Don't hesitate to reach for others for help! For example, in my case, talking with number theory professor and my father greatly clarified things and speed up the process of learning.
Well I feel that I should stop myself. Sorry for turning this forum into math forum 
I just love math, so I used it as an example of the complicated thing, some parts of which can be harder that other, but you still can master those hard parts with constant efforts and persistent attentive thinking... and I carried away too much...