Any relevant research in Motivation?
Understanding people better can enable you to more effectively teach them.

Taken as axiomatic, this can be a helpful thought. It can also be a distraction. The value depends on how it's applied. When trying to understand "people", you can aim to understand people as a collective of unknown size, from unknown backgrounds, and unknown individualities; something you cannot ever "know" or understand. Conversely, you can aim to know the 35 students in your classroom, with backgrounds, experience, limitations, and gifts which you can know, and whom you can begin to understand. At least with respect to how that information affects their performance within your subject matter.

I have no links to research into motivation, and can only suggest that if any were found it would be inefficient to read it, or the conclusions drawn from it. The results would be extrapolations of an imperfect model using statistical analysis gathered by researchers with some pet theory to prove, or disprove. The 'repulsive' statement you found is merely one example of many which can be "concluded" from some observation. While, within the experience of the writer that statement might be true, it could be also be untrue in others' experience. What's left out of the statement are the conditions which define "fail", what consequences there are for failure, what is expected to be gained by success, and how much effort is required to achieve what "change".

If I will be passed over for a promotion unless I gain 6 inches on my biceps and increase my bench-press limit by 100 pounds, and the promotion has a net increase of 15% in pay, I'd personally be willing to "fail" rather than change. Given a different set of conditions, 3% pay increase for learning a new Java framework, I might not consider "failure" as an option.  (I don't know Java, so I'd have to learn that first.) In the second case, to me, the change is more desirable, even though the consequence of failure is one-fifth that of the first.

People, as an amorphous mass, can be classified, categorized, and "pigeon-holed" neatly by any study or research project. People, as one-on-one real individual humans, seldom fit into any neat "system," no matter how well researched, or refined it may be. If a system were ever developed that could classify people, manufacturing lines could be reduced to producing the one version that fit the needs, and wants, of everyone. We would have the 'one' computer system, with the 'one' OS that everyone could use to do everything they needed. The 'perfect' automobile could be made in the correct color, black according to the legend of Mr. Ford, and the Big-Four would be out of business. And, in your case, you could write to last, perfect, syllabus and lesson plan for each course you teach. (I'll not hold my breath for any of these things.)

Interestingly enough, I did have some college classes, and I somehow match part of what you described. I did not know what I wanted to major in. I never did manage to "focus" on the core courses. Nor have I managed to collect so much as an Associate's in anything, despite having nearly 150 semester credit hours collected. (I did manage to earn, by accident, a "certificate" for Linux Professional, which surprised me when it came in the mail after I failed to collect it from the office.) I've been connected to computing, and electronics, for decades, and will always be doing something within those areas. Yet I was not sure that should be my major. Perhaps it would be better to major in business, and use my computer skill within that arena? Of course, it didn't help much that I viewed computers as "fun" either. When ever I do "work" with computers it feels like being paid to play, and I almost feel guilty collecting the check.

The "motivation" you seek to understand exists in everyone. It remains individual as to what can activate, or trigger that motivation, or which subject(s) it might be applied to. The target of motivation may not even be a goal, it could be a journey. Some may be motivated to achieve some tangible reward, or to avoid some imagined consequence. Others could be motivated by the "rush" of doing something, or of experiencing the stillness of doing nothing. 

That you don't see, or recognize, the motivation of another does not mean it does not exist. "Absence of evidence is not evidence of absence." (And there ends a 3 hour tour into the rabbit hole of The Demon-Haunted World: Science as a Candle in the Dark.)