I'm currently a graphic design student in my junior year, so I'll throw my two cents in.
I have a friend who is just like you. He went to a high school whose curriculum was based around the arts. He's been doing graphic design stuff for around 6 years, probably about the same amount of time you have. He feels the same way as you do in regards to certain things, especially the "know-all" mentality. He does, however, admit that he doesn't in fact know everything. His mentality is that you should never think that you can't learn anything new.
First, I'd say I entirely agree with lxskllr. And don't get me wrong, I don't have the "know-all" mentality as you say. I know there is much to learn, but as far as this field - I have self taught myself and feel like I have more freedom to try new concepts and experiment than if I was stuck with a regular college diploma. College is great, and necessary for some people - people that need options, and want to try out various courses to find their path in life. But I have already found mine, and as I said I will go back one day.
As far as life lessons, art history, and general art "knowledge".. Like I said, I have been drawing/painting since I was 6 years old, and attended private sessions and lessons from various profound artists in my country as well as in the states. I have tons of books, and inspiration to look at. I'm not the type of guy who learned a program, found something that looked good, and cloned it over and over again for different clients. Its more of a passion than it is a job, which is why sometimes the money doesn't concern me as much as facing a new project or challenge.
I spoke to my father about this today. He seemed to understand, and I will probably go through this semester, finalize the year, and see where the company takes me during that time period. After that, I will take a break from school, which will give me that much more incentive to grow my business with the "this is all I've got, and i better make it work" mentality - you work that much harder.
My father graduated with a diploma still hanging in the hall of fame at the institute he studied in - being accepted to study abroad on a full scholarship during communist times (pretty hard to go abroad back then *legally) as an aircraft engineer for 6 years. He was left with no job once everything began shutting down, left to start his own business that had nothing to do with aircraft (but had everything to do with a passion he's had since a child) so he can support his family. I'm not anti-college, don't get me wrong, I just think that I should go there when I feel the time is right.
Most of my friends have been attending universities for longer than I have, not knowing what their "talent" is, or what they want to do in life that makes them happy. Because one day the 100k salary won't be enough. Looking back at life, knowing you've been going to the same office, same time, same days, you ask yourself "what am I going to leave behind?" Which is why it is important for everyone to find their way early in life, and College is a great place to do that, its just not the case for my situation.