Coaches and Mentors
The greatest teacher is life but that might be too slow and too general for most. Personal coaches and mentors in the form of another person who's been there is probably the most accelerated learning source one can encounter. They provide one-on-one feedback on their area of expertise.
The trick to a great mentor is to find one who has gone through what you are going through right now. The more this mentor has in common with you, the more they can help you.
However, they are hard to find and there's no where to sign up for mentors directly..
Books and Reading
The next best thing is books.
I find the concept of written language mind blowing. Books and words contain the thoughts of a stranger. They allow the transference of experiences and lessons learned and they transcend time. You can see exactly what life a couple of hundred years ago was like by flipping open a book. You can live vicariously through the words. In a way, you can experience the lessons from many lifetimes by reading.
It is the closest thing to telepathy next to speech. It's a very dense and structured form of learning. The written history has thousands of years of record. Someone has lived the problems you have and recorded their solutions. It's up to you to find their written words.
Although most of us haven't been alive for too much, we have learned a lot in the course of our lifetimes, no matter how young, to be able to draw many lessons from. This is one of the reasons I keep a journal diary. I can look back on my life a week earlier, a month earlier, a year earlier and reflect on how far I've come since then. I think back to regrets, hangups, and mistakes and try my best to learn from them. Writing them down really helps me get crystal clear about them. And it's fun to just look back at the memories.
My roommate found a fortune cookie that said, "The faintest ink is better than the clearest memory." I agree wholeheartedly.
I am so immensely grateful to be alive in this day and age because the Internet exists. There's is just so much information out in the World Wide Web. Anytime anyone asks me a question I don't know the answer to, 90% of the time I am confident I'll be able to find the answer on Google. Effectively, the expertise and knowledge on the Internet effectively has become an extension of myself.
I have a biomedical engineering degree but became a web/software developer because I love learning and fell in love with how the information about the latest and greatest in the field is available for free, without expensive tuition and research grants and million-dollar equipment like the biomedical engineering field. This was only possible because the Internet provides information openly for free and tools like Google, Youtube, and massively-open-online-courses exist.