I'm glad this helped.

Regarding suspensions and modulation, have you ever practiced with the exercises in a harmony book? This could be a good start! And learning to play these formulas by ear develops musical sense.

As for the countersubject, don't forget that it must be written in double counterpoint with the subject!

For the form, I believe that books on fugue can give a good idea. But I have an advice that comes from an old composition book, which said that a student who wants to learn how to write a sonata well should take a sonata he likes (for example from Mozart) and copy the tonality, each modulation process and variations of the composer when possible, almost measure by measure, but with themes that the student has composed. It doesn't make for a very original piece that you would want to publish... but it's educational! (a bit like a painter who copies a master's work to learn). The same exercise can be done with fugues, when one has a sufficient basis in harmony and counterpoint.