It's a big topic because "form" can relate to genre. Classical form is easy enough to master. It's when you get into contemporary styles and/or "through composing" where things change.
Your piece is basically tonal. In classical forms different melodies are often linked by "bridge passages" that usually involve changing key. It's a good place to start (because even if you write in a more modern style you need some transition to get to a new melody). Sometimes there's no bridge passage - pieces referred to as A-B-A etc where melody A ends in a cadence followed by B. Then when B has done melody A returns. Minuets with their trios are often like this.
Can I suggest therefore
1) that you study a few classical pieces: Mozart's Eine Kleine, Or if you're not happy with orchestral, try Mozart's Sonata in C, the famous one. (Mozart was quite good at music so he's reliable 😄 )
2) Learn enough theory to modulate to a related key - the dominant, subdominant, relative minor (submediant). You can save the surprise modulations for later! using diminished chords, the Neapolitan 6th, Augmented 6ths and so forth. (Again, the first movement of that Mozart Sonata is full of hints.)
3) Map out your phrasing. At first go for 2 or 4-bar chunks.
4) Study a few through-composed pieces - just intelligent listening may be enough.
I suffer your pain - my sense of melody was almost killed by my partial college education. Thankfully it's gradually returning - and in a way I'm not unhappy because I don't want to compose rehashes of the classical era. So most of my stuff is through composed. To me, form is a matter of balance, proportion and contrast.
Good luck.