I spent many days scoring out things like Bach's Brandenburg concertos
That's one of the best ways to learn. Decipher the mysteries of the songs you like. The only thing that stands in your way of learning and becoming a brilliant musician is fear of the unknown. Clearly why you've asked this question.
I'll venture that I'm a professional musician. My main (but not only, I'm an artist as well) source of income comes from teaching music to really little kids. But as I've learnt over the past four years that I've been doing it, is that music is really a "people" job.
So here's my experience so far. I never set out to become a "professional" musician. In fact I had a very utopian vision of merely hosting artists, musicians and like minded people in a kind of a learning centre/ hang out place. Since I also was of the opinion that no university would have that specific course to cater to me, and that I didn't want to go into debt the first thing out of school I decided to get "job experience".
Taking what I had learnt in school- how to read music, I went to the UCT music library and photocopied lots of random music, learnt it and put together a repetoir of an hour(memorised). I stepped into the first bar/ restaurant asking for a job to play in the evenings, and ther you go.. and then the next one... so two gigs a week for two hours each kept me going for a while (I live very frugaly).
I figured a course in teaching guitar would be usefull, so I looked around for a correspondence course and settled on one from Trinity College London. I couldn't go straight to the teaching licentiate, so I did the Associate in performance. By the time I got to doing the licentiate I was a little disillusioned. The associate had such a predictable, boring repetoir, my invidulator was actually a saxophonist and the licentiate really just set me up to be a promoter of the Trinity College "franchise". I like to challenge things and look for better alternatives in learning, but this course was as stale as stale could be. The course was also so simple- teach. And then when I'd finnished teaching they'd tell me whether I'd be able to teach more. No kidding. I was also instructed to buy all their books. ALL OF THEM.
So I said "stuff that". By this time I was looking further afield than down the road for places to play. I was cold calling hotels, made a demo cd, put together some kind of a promotional flyer and did a bit of "PR". Like I said this is a "people" job and I might have done better if I was a bit more confident, or just thicker skinned. I learnt the valuable lessons you learn when freelancing, contract contract contract. Do not trust people, just don't. Moral of the story really. Moving along though, before I scare you to death. I tried to go the route of agents, and I got one bizarre call to go play in ....Zambia. Yup, alll the way up there. My passport had expired though. A blessing in disguise I'm sure. So the agents didn't really work out. I did put an add in the local newspaper as "classical guitarist" and got the blessed job at the old age home. They paid my regular hourly rate, and even though I had to get a new repetoir for them it was valuable in learning how to "work a crowd".
After getting a few regular gigs in hotels I got to make some fairly decent money, but I was SO TIRED of my repetoir by now. I would sometimes drift off half way through it and I asked myself, are you stuck or what? I was so tired of performing that I didn't want to learn anything new. Then I heard about the job teaching little kids and I thought it was just the change of scenery I needed. And I have learnt SO MUCH. Not that I recommend that you take up that kind of job, but I think it helps to see music as a practical thing, something you share with people, not some abstract acquisition of skills. Just today I had so much fun showing them a marimba. It's a very badly tuned one, so there's a lot of dissonance. But I thought, "let's see how they respond to it". Strangely enough they actually managed to get some good sounds out of it and resolved the dissonances beautifully. Largely becuase they don't have the expectations you have from playing perfectly tuned instruments. But I'm waffeling now.
I joined this forum and it's very helpfull in the "current" aspect. You get to hear what the trends in recording are and other practical things that you just don't learn in the traditional music classroom.
Although in hindsight, if you are looking for a carrer in music, and from what I've heard at the Cape College of Music performances, you can learn some really good things at a musical institution. It's NOT absolutely the only way, but it'll be a good experience no doubt. And like I said twice now you get that all important "people" exposure, meeting the right people, mixing with the right people. Hope that was some help, I'm not the next thing on the Boere musiek top 20, but I do love music. Maybe after an accumulation of colourfull experiences I'll have enough to put my master plan into action, if I still want to in the end. At the moment I'm content with composing and feeling that rush of megalomania when you write something you like.