I have Rimsky-Korsakov's "Principles of Orchestration", very good book with lot's of info. I read it long time ago but wasn't able to remember all, so much!
I bought last year "The study of Orchestration" by Samual Adler, I read it and use it now for looking up things
Samuel Adler's book is a very good reference - so are the Rimsky-Korsakov and Berlioz/Strauss books, though their age do shine through.
I wrote the horn high because I want it to be intense emotional, (like high cello/bassoon) but didn't think about the difficulty with the dynamics. Would it be easier if the horn reached that high note coming from lower, like a scale or so? Or will it always be uncomfortable to play?
The horn part is in my opinoin just right - it's not too high. Though Contratrombone64 is right in that it's at the edge of what is considered "safe" for amateur players. But this part shouldn't be a problem for a decent amateur, nor should you, unless it's the whole point, aim for writing "too safe".
I don't know all that much about the clarinet break, will have a look into that.
The "break" isn't really that much of a problem in this orchestration.
A general note on writing for the orchestra - which I actually do think you're adhearing to quite nicely. - Don't treat the orchestra-"instrument" as you would a piano. Each of the instruments in the orchestra has it's own "idiom"/life, both solowise as well as in tutti/accomp. parts.
You'll have to be very "precise" in your way of thinking and writing for the orchestra not to let the piano-idiomatics shine through
Some quick notes on your orchestration:
The oboe, though nicely written, tends to be a little "harsh" in the lowest region where you've written your first theme - especially if played by amateurs.
The strings: why aren't the notes (d and a) in v2 and vla at measure 1 not tied together with the 1st notes in measure 2? Generally your string writing is good! When writing for the strings you'll also later have to think about phrasing, with bowmarkings. When writing for strings in combination with other instruments, historically (classic period to the romantic period) the strings and the winds compliment eachother in that not all of them play the melody/rhythmic lines at the same time. The passages from measure 25 and 42 (with upbeat) seems to me to be less idiomatic in that you've "enhanced" and focues on the piano-idomatics rather than writing the music.
The brass: m18-22 - very nice, though the doubling of the trombones strikes me a little odd. And why is the oboe doubling the trumpet here?
The ending sounds (melodically) a little odd/empty to me.
Keep on the good work!