I've identified a common, huge problem people have. They struggle with indirection.
They want Z. They find out that doing W will help them figure out X which will help them solve one problem with Y which is a component of Z. But they don't care about W much. They wanted to deal with stuff more directly related to Z. At every step in the chain of indirection, their motivation/interest/etc drops off significantly.
This ruins their lives.
Indirection is pretty much ever-present. Doing things well consistently requires doing some other stuff that's connected to it via several steps.
Say you want to be a great artist, but you're bad at English. This gets in the way of improving at art, e.g. by discouraging you from reading art books (reading is a difficult, slow struggle for you) and causing frequent misunderstandings of the content of art books and lecture videos. Do you then spend significant time and effort improving at English in order to improve your art? Many people wouldn't. They wanted to spend time working on art. They like art but not English. They're relatively rational about art, but not about English. And they suck at indirection. They do things like forget how working on English connects to their goal of making progress at art.
A lot more indirection than this is typical. When working on English, they will run into some other problems. While working on those, they'll run into sub-problems. While working on those, they'll run into sub-sub-problems. They'll need to solve some sub-sub-problems to make progress on the sub-problems to make progress on the problems in the way of English progress to enable making more progress with art books.
Sub-sub-sub-problems often get into philosophy and some other generic issues. They are bad at learning. They dislike criticism. They have problems with emotions. They aren't very precise or logical. They're biased rather than objective. They don't understand effective methods of problem-solving. They aren't persistent and just want things to be quick and easy or they give up and look for something they find more intuitive and straightforward. They are too "busy" or "tired". They are directing a lot of their effort towards their social life, and getting along with people, rather than to problem solving. etc, etc, etc
People are fine with indirection sometimes. They want a cookie, and they spend time reaching for a cookie jar and opening it, rather than only directly eating the cookie. That bit of indirection doesn't bother them.
One reason people have a problem with indirection is they have little confidence in their ability to complete long range projects. They don't expect to get to a positive conclusion they can't reach very quickly. They have a long history of giving up on projects after a short time if it isn't done yet. So any project with a lot of steps is suspect to them. Especially when some of the steps fall outside their primary interests. A physicist will work on a 20-step physics project, and if he doesn't finish it's ok because he was working on physics the whole time. But he won't work on learning philosophy of science in order to do physics better because if he doesn't complete that project (not only learn useful things about philosophy of science, but then also use them to make physics progress) he'll be unhappy because he enjoys physics but does not enjoy philosophy of science.
A major reason people suck at longterm projects is because their lives are overwhelmed with errors. Their ability to correct errors and solve problems is in a constant state of being overloaded and failing, and they end up having chronic problems in their lives. There are other reasons including that people have little clue what they want and that they have little freedom for the first 20 years of their lives so they can't reliably pursue longterm projects because the projects are disrupted by the people who control their lives (especially parents and teachers).
People also, frequently correctly, lack confidence in their own judgement. They think there is a chain of connections where they work on W to work on X to work on Y to get Z. But they don't trust their judgement. Often correctly. Often they're wrong over and over and their judgement sucks. It requires better judgement to deal with indirection. People with bad judgement (almost everyone) can have somewhat more success when focusing on limited, easy, short projects with fewer layers to them. But that's no real solution. The structure of life involves many connections between different areas (like English skill being relevant to being an artist, and philosophy skill being relevant to being a scientist) rather than being a bunch of narrow, separate, autonomous fields.
Pursuing problems in an open-ended way often takes you far afield.
One of the other issues present here is people have limited interests, rather than open-ended interests. That's really bad. People ought to have broader curiosity and interest in anything useful and important. One of the reasons for such limited interests is most people are really irrational with a few exceptions, so their interests are limited to the exceptions where they are less irrational. This gets in the way of open-ended problem solving where one seeks the truth wherever it may be found instead of sticking to a predetermined field.