I'm impressed at how measured and steadily you're progressing as a composer. I'm more impressed that you're starting by studying theory instead of just diving in. When I started, I was intent on composing a masterpiece every time I sat down, which is a great way to disappoint yourself. Keep writing, keep learning, but never be limited by your own experience. The more you experiment, the more you'll learn to put all this theory into practice, and the less your compositions will sound like exercises (and more like music!).
I like the direction you took with this most recent piece, and it presents a great opportunity to learn something useful about harmonic writing. It won't make much sense if you're not familiar with "intervals", but I'll leave that up to you to research. It generally doesn't sound "good" when two notes that are a fourth or fifth apart move the same distance in the same direction. That's called parallel motion, and it tends to make the music sound like a Gregorian chant (ie. from before anyone had heard of tonal harmony). The violin melody is an example of this. If you're familiar with solfege, a fourth is the distance from Solup to Do (or Do to Fa), and a fifth is the same distance as Do up to Sol (or Fa to Do). Since it seems like you're studying scales, it's based on the distance from the root of the scale. In a C major scale, F would be the 4th scale degree, and is thus the innterval between C and F is a fourth.
I'm just rambling at this point, and you might well already know all this. I'm just bored and never get the chance to be a music dork anymore =/ Forgive me.