How is motivation the key to one's success in school when seeking a higher education?
Many different factors contribute to success in each course you take in school including natural ability, preparation, effort, and motivation.
If you never completed high school math, it's unlikely you would succeed in an advanced university level calculus course no matter what the level of your motivation, simply because it would be too difficult to make up for several years of missing prerequisites over the course of a term. Secondly, just as most of us can pick up and throw a football a few feet but don't have the talent to be star quarterbacks, there are variations in our abilities in academic subjects.
Nonetheless, the single most important factor in achieving passing grades is probably motivation, because whatever your level of talent and preparation, if you show up to class every day, pay attention, do your homework, complete all your assigned readings, and turn in all assignments on time, you are much more likely to do well than a student who does not do these things.