Hey man. There's a lot to like here, especially for a first orchestral effort! Here are some of my thoughts on the instrumentation, but know I'm no expert and I can't really say anything definitive.
M. 14-16: Not sure you need the 2nd oboe doubling with clarinet and trumpet. It goes into the very bottom of the oboe range, which isn't really idiomatic for passages in lower dynamics.
M. 20: I would rethink doubling the 2nd violin trill in the trumpets and horns. It wouldn't be possible to do a half-step (F#-G) lip trill, and valve trills would have a chance of really muddying the sound there. That may be fine for your purposes, but you'd probably get a cleaner result from clarinet or oboe.
M. 26-30: The 2nd flute line would probably have serious problems cutting through the rest of the dense orchestration. I get it's doubling the trumpet, but it's almost pointless because the trumpet would completely overpower it
M. 40-47: The oboe doubling is again dipping a little too far into its low register. Generally, anything below the staff isn't very characteristic of the oboe. I would probably reassign that line to a clarinet, or have the oboist switch to english horn for that line, which I think may be more the sound you have in mind.
Some general thoughts: The best advice I've heard for instrumentation is that your choice of instrumentation should clarify the form of the piece. I think you're getting this idea, since you chose to really cut back on the brass for the second theme after they had largely dominated the first (which is itself pretty unusual compared to what I would typically expect from orchestral writing). It seems like you get that idea, but I'd really encourage you to think about how the way you choose to orchestrate certain sections of the piece helps define how they relate to others.
One last minor critique: It strikes me as odd that you have a long pause inbetween the first two themes. I really felt like the piece lost a lot of momentum and I really think it would be preferable to have a smoother transition between them. I hope I've been helpful and the things I've pointed out make sense. Forgive me if I've written anything strange, it's kind of late and I'm not thinking 100% clearly.