Reddit mentions of The Study of Fugue (Dover Books on Music)

Who the hell told you counterpoint was easy to learn?

No, my friend, counterpoint is the pinnacle of composition. It's dreaded by every music major and even the theory/composition majors have a healthy fear of it.

I don't really know of any online source that can teach you what you need to know, either. Maybe some theory, but not counterpoint. You need to get a book and diligently work through the exercises. I recommend Study of Counterpoint: From Johann Joseph Fux's Gradus Ad Parnassum, Kennan's Counterpoint and Mann's The Study of Fugue for this. Check out both the Fux and the Kennan first and decide on one, then if you find that you want to go deeper go ahead and get the book on fugue study.

Depends on what sort of rep you're in to (into?). I read a lot of popular music scholarship. These are books that I have either read or am going to read, in no particular order:

Brad Osborn - Everything in its Right Place: Analyzing Radiohead

Robert Walser - Running with the Devil:Power, Gender, and Madness in Heavy Metal Music

Walter Everett - The Foundations of Rock

Walter Everett - The Beatles as Musicians: The Quarry Men through Rubber Soul, Revolver through the Anthology

† Understanding Rock: Essays in Musical Analysis, ed. John Covach and Graeme Boone

Richard Middleton - Studying Popular Music

Allan Moore - Song Means:Analysing and Interpreting Recorded Popular Song

†Analyzing Popular Music, ed. Allan Moore

† = A bunch of essays crammed into a book. These are nice because you can read a 20-50 page study on a topic and move on.

I'll recommend these as more general reading, geared toward classical music but useful in other styles as well:

William Caplin - Classical Form: A Theory of Formal Functions for the Instrumental Music of Haydn, Mozart, and Beethoven (Every researcher in the field knows or should know this one. I would also recommend Hepokoski and Darcy - Elements of Sonata Theory, which is the other big form book, but it is a veritable tome and hardly light reading. Not that much of what I've recommended is very light.)

Janet Schmalfeldt - In the Process of Becoming:Analytic and Philosophical Perspectives on Form in Early Nineteenth-Century Music (I haven't read this yet, but it's in the same universe of form studies as the above.)

Kofi Agawu - Music as Discourse: Semiotic Adventures in Romantic Music (Agawu has written a lot on various topics and should be on your radar.)

Alfred Mann - The Study of Fugue (Broken up into two parts: part 1 is a history of fugue, part 2 is about fugal technique.)

I'll point you toward this one since it is totally awesome and helped me to understand medieval liturgical music theory and culture (though you might find yourself lost if you are not already familiar with some of the concepts):

Anna Maria Busse Berger - Medieval Music and the Art of Memory

You might also consider ordering a hard copy of some journal issues, as these can be quite compact and give you a lot of breadth.

Journal of Music Theory (JMT)

Music Theory Spectrum (MTS)

If you would like to end with Chopin, you only need to study tonal theory. So twelve tone topics are not of any use since that topic is 20th century, after tonality.

If you didn't do voice leading (SATB harmony): Are you interested in voice leading? If you want to get to the more advanced topics of tonal theory, you'll need to cover that. If so I would suggest this book:

https://www.amazon.com/Musicians-Guide-Theory-Analysis-Third/dp/0393600491/ref=sr_1_1?s=books&ie=UTF8&qid=1535936804&sr=1-1&keywords=clendinning+theory

Have you done species counterpoint? Species counterpoint will be very helpful in dealing with just about all music. I would recommend Fux's book:

https://www.amazon.com/Study-Counterpoint-Johann-Joseph-Parnassum/dp/0393002772/ref=sr_1_1?s=books&ie=UTF8&qid=1535936929&sr=1-1&keywords=fux+counterpoint

If you've already done species counterpoint: For more advanced counterpoint (not useful for Chopin, but necessary for anything with fugues in it, obviously) I would suggest Mann's book:

https://www.amazon.com/Study-Fugue-Dover-Books-Music/dp/0486254399

For a complete discussion of forms I would suggest Berry's book:

https://www.amazon.com/Form-Music-2nd-Wallace-Berry/dp/0133292851

For an in depth and modern discussion of sonata theory (remember that symphonies are also often times in sonata form), I would suggest Hepokoski's book:

https://www.amazon.com/Elements-Sonata-Theory-Deformations-Late-Eighteenth-Century/dp/0199773912/ref=sr_1_1?s=books&ie=UTF8&qid=1535937360&sr=1-1&keywords=hepokoski+sonata

If you already know species counterpoint and voice leading you can study Schenkarian Analysis. For this there's two books I would suggest:

https://www.amazon.com/Analysis-Tonal-Music-Schenkerian-Approach/dp/0199732477/ref=sr_1_1?s=books&ie=UTF8&qid=1535937496&sr=1-1&keywords=schenkerian+analysis

https://www.amazon.com/Tonal-Analysis-Schenkerian-David-Damschroder/dp/0393283798/ref=sr_1_2?s=books&ie=UTF8&qid=1535937488&sr=1-2&keywords=schenkerian+analysis

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If you're interested in composition, that's the other side of the coin and so all the above are of limited use. Let me know if you want books for composition.

Some of my favorites:

Messaien - The Techniques of my Musical Language

Straus - Introduction to Post-Tonal Theory

Ross - The Rest Is Noise: Listening to the Twentieth Century

Chinen - Playing Changes: Jazz for the New Century

Gottschalk - Experimental Music Since 1970

Perle - Serial Composition and Atonality

Xenakis - Formalized Music: Thought a d Mathematics in Music

Mann - The Study of Fugue

I believe anyone studying counterpoint should read Johann Fux's Gradus ad Parnassum, nowadays published as two separate volumes translated and edited by Alfred Mann, namely The Study of Counterpoint and The Study of Fugue.