In my years, I have had three major areas of study: music, martial arts, and writing. I find myself incredibly fortunate to have all of those experiences, because they all benefit each other (not unlike my three problems).
These three disciplines are all arts. In this sense, they are all extremely similar, and because of that, I can use the knowledge and principles gained in one field to aid in the others. Even if I do not have a master or teacher giving me all of my lessons, I have enough knowledge and experience to figure out what is wrong and discover how to fix it. In any given art, the other two arts are my teachers, or at least my helpful colleagues.
Although these three disciplines are so similar, they are not the same. Significant differences exist. Music and martial arts are very physical activities, whereas writing is not so much. Writing and music are largely prepared activities, whereas martial arts are spontaneous in their execution. (At some point, I will do a more thorough list/diagram of them.) Because of these differences, I cannot simply use all principles from all arts willy-nilly.
The world is full of examples. Being able to approach these examples from different angles, with different eyes, will allow you to get the very most out of the examples you find. It will also allow you to realize that there are more examples than your one discipline may have you believe.
Most people already have a good chunk of knowledge about one or two subjects already. What do you know? What do you spend the most of your free time on? What could you teach a 14-year-old child if they asked you for knowledge? Those are the disciplines that will add to and blend with your writing.
And if you don't think you have a specialty, that means you know a bit about EVERYTHING, so you have zero excuse to not be doing this.