Hey!
Great to hear you are interested in composing and are writing pieces. That's step one. To get good at composing you will have to write A LOT, and a lot of it is going to be stuff you think is crap that nobody wants to hear, weird experiments that don't work. That's part of the process and totally normal. Just keep writing and evaluate your work against what you are trying to accomplish. And don't forget to have fun and enjoy yourself even if the result ends up a dud sometimes.
You are asking some pretty big questions. There is no objective right and wrong in composition anymore - essentially, if it sounds good to you, it's right - so the answers will depend on the style you are trying to write in, and what goals you have. That makes it hard to give feedback on your piece, because I don't know what you are aiming to accomplish. From what I can see and hear, you have learned many things about harmony already.
If you are aiming to write in the classical music tradition, you should look into studying:
- Harmony and harmonic functions
https://en.wikipedia.org/wiki/Function_(music). The article mentions two systems, the German system where you write the tonic with T, dominant with D etc, and the Vienesse system, where you write roman numerals. Pick one and look up courses, start studying common chord progressions and how they are used. This will give you more tools for constructing your music and coloring it to make it more interesting, more creative ways to modulate etc. I prefer the German system over roman numerals because to me it tells you a lot more about WHY chords fit together once you start getting hang of the subtleties of it.
- Counterpoint. This will teach you a lot of good tools for writing melodies and fitting them together with harmony, handling dissonance, voice leading and lots more. Start with Species Counterpoint (where you progressively write more advanced counterpoint over a Cantus Firmus) and go from there. I'm sure there are good courses out there.
https://en.wikipedia.org/wiki/Counterpoint#Species_counterpoint- Study and play a lot of music that you like and try to figure out how to emulate what the composer did and why it works.