unfortunately as several of us can testify from our own experience, just making an effort, even by someone interested capable and healthy, does not always lead to success. The effort has to be directed correctly, and it takes time to learn how to do this. As a college professor for some 40 years, I almost never had a student who I thought "could not" have succeeded in my classes, if they had behaved in the way needed for success. I.e. only a handful out of thousands of students seemed ill advised to be there. However the actual success rate was extremely low, e.g. passing rates in calculus were often below 50%, even with generous grade inflation.
The number of students who simply attended class regularly (i.e. basically always), read the book, handed in assignments, asked questions in class and came to office hours, were usually numbered on one or two fingers out of a class of dozens. Students who actually entered class knowing the stated prerequisites were almost non existent, and usually limited entirely to foreign students. All my students apparently thought they were trying hard, or as hard as should be expected, to pass.
A (should be) famous study by Uri Treisman at Berkeley, followed a group of racial minorities who for some unexplained reason were failing miserably out of calculus, even though they did many of these things I mention, and it was found they lacked other more subtle study skills, like working in groups, challenging each other with the hardest problems, and (I would recommend) reworking tests they had already taken to be prepared for the same questions on the final. When Treisman taught them these skills and organized a study group for them, the same group of minorities became the stars of the class.
http://www.utdanacenter.org/about-us/staff/p-uri-treisman/
The same change of study habits worked for me, from a first failing experience in college to a later honors level one. When I attempted grad school, I also found that working together with others was extremely valuable, and I eventually succeeded in my goal, long delayed, of getting a PhD, although that was the hardest thing I ever did, and my advisor was a huge help.
In my opinion, most people who are not clearly unqualified, can succeed in undergrad and even grad school by employing the right techniques. It is not so clear to me that anyone knows how to predict just who can produce interesting thesis research at the doctoral level or beyond. No doubt there are also techniques that work to assist in research too, such as reading the work of top researchers in the field and trying to prove the results oneself, or generalize them. One is often confident that ones brightest students will succeed in obtaining a doctorate, but even then, the apparently smartest ones do not always produce the most interesting research. Imagination and originality are somewhat hard to measure reliably, and they do differ from technical power, although without that it is hard to finish a project, no matter how well conceived.
Come to think of it, Uri Treisman refined his program to apply also to doctoral level studies and went on to produce a large number of successful PhD's.
But as far as just learning to appreciate advanced math, this is a project anyone can enjoy, by beginning at the bottom, and trying to understand elementary, but significant math, such as Euclid's geometry, or number theory.
