If you want to remain self taught, you need to embrace at least two very important ideas:
- embrace an very, very critical eye for self examination
- test all you notions of quality and correctness against extensive evaluation of the great composers and great teachers in the styles you are studying.
When it comes to part writing knowledge, one of my first questions is: how many of the Bach 371 Harmonized Chorales have you played and analyzed, and even better yet, sung the various parts?
I'm self taught too. When I make a claim about musical aesthetics or technicalities, I like to be sure I know I'm aware of that thing in the music of composers like Bach, Mozart, Chopin, or Debussy, depending on the particular style. And, for what it's worth, I do not like folk who hunt around for one cherry picked example, often applied out of musical context, to bolster a weak claim.
I think you should really develop enough knowledge of a style to be confident you know the details. Then you pick from one of many examples, or perhaps a favorite example, to illustrate a point.
So, study the scores of the greats as extensively as possible, and integrate that with textbook study and exercises.
When you do an exercise, do it not once, but 2 or 3 versions, then critique your work to select the best one, or at least to find their relative merits and faults.
One final thought: make sure you consider the reason, the why, of various musical principles. For example, "don't double the leading tone." Why? Because if a doubled leading tone is resolved properly, the two parts will move in parallel unisons or octaves. Why don't move in parallel unisons or octaves? Because it destroys the independence of the parts. Why make parts independent? Because one of the primary aesthetics of contrapuntal part writing is melodic variety between the various parts.
You don't have a teacher to guide you, so you have to dig, dig, dig to get to the deep knowledge of the subject.