As usual bandcoach is on point... The trouble with using the internet as a knowledge base is that there is little or no quality control on what is posted. It's all about your own knowledge and level of research... Just like doing research in an actual library, you have to know how to find and access the relevant information. Just because someone says you should do something on a YouTube clip, doesn't make it true or useful!
To answer your question: the best learning resource for mixing/mastering is to assist or learn from an experienced and talented mix engineer or mastering engineer. That's why up and coming engineers all want to be runners in big studios. The trouble is that those studios and opportunities are few and far between these days as studios are dropping like flies
So the next best thing is to find information written or moderated by experienced and talented mix engineers. As an experienced engineer myself, a few of the best and insightful mix engineer resources I have found online are Dave Pensado's Into The Lair on Pensado's Place, or mix tips from Michael Brauer. Not only are these two guys very good engineers, but I also find them to be very open and good at explaining their own techniques.
You could also go down the route of buying a respected book on the subject which will help in different ways, especially in laying the foundations of the theory and methods involved. That will probably cost around £25 - £30 which isn't exactly pricey.
Either way you should be structured about it. Make a list of areas you have problems with and address those through research. Make notes and come up with ideas and things to try. Be pro-active.
Another way is that you can get an experienced mix engineer to teach you, if you can find one and if you have the money. There are special seminars, and even things like Mix With The Masters where you spend a week in a studio with a top mix engineer. Strangely enough that happens to be one of the things I do, so do check out my website
Also look at the blog on my website as a resource, there's a couple of articles on mix techniques on there that may be of interest as well.
Good luck.