If you spend 5 years only playing your favorite parts of your favorite songs on a guitar, you're not going to be a good musician.
Just like if you spend five years doing the fun parts of programming or engineering or history, you won't be a good programmer, engineer, or historian.
College (or an apprenticeship, or trade school) isn't required to learn the hard/boring parts, but very few people are capable of doing so on their own at that age.
If you started to self-direct at 13 (or earlier), you'd probably be pretty self-disciplined at 18.
I would even speculate that how interesting / boring you find something is not static, but that otherwise boring things can start to seem interesting once it's clear that they are relevant to something you want to achieve.
Personally I learnt math mostly from self-study, and I've learnt it well enough that I now have published research (see [1], also [2] is in review and has been recommended for acceptance with revisions by the referee), have been invited to give talks about my research [3][4][5], and have stayed for a few months as a visiting researcher at University of Muenster [6]. I don't consider myself as having a lot of self-discipline; I have a tendency to hit the snooze button many times, to overeat, to let my room get messy, etc. But once I have an idea, I want to flesh it out and share it with other people, which motivates me to do the things that are necessary for this, which are sometimes boring.
[1] http://content.algebraicgeometry.nl/2017-2/2017-2-007.pdf
[2] https://arxiv.org/abs/1612.07306
[3] https://www.uni-muenster.de/FB10srvi/HotNews/show_artikel.ph...
[4] http://www2.math.binghamton.edu/p/seminars/arit/arit_spring2...
[5] http://www2.math.binghamton.edu/p/seminars/arit/arit_fall201...
[6] https://web.archive.org/web/20170121000748/https://wwwmath.u...