Hey Nom, thanks for posting this! I can totally understand where you are coming from. Orchestration can often be the most challenging part of writing music, so I can definitely appreciate the work you've put into this. It's taken me years to get to a level where I feel comfortable with orchestrating and part-writing, so I think it's great that you are reaching out for feedback.There were a few things that I wanted to point out that may help improve the piece:
Mixing/mastering: I feel like a lot of beginning composers/arrangers/remixers never use reverb. It sounds like you do have reverb in the piece (which is great!), but it may be a little too much. I think that's contributing to the "muddiness," especially in the drums and strings. I'd reduce the amount of reverb you have applied to the piece, which may help clear things up a bit. It may also help to look into adding some light compression and reducing your levels to prevent peaking (which sounds like it happens when things are trying to get loud.) You should try this step first to see if this helps before looking into my other suggestions.
Brass: With the trumpet hits in the beginning, it sounds a little "weak" due to the register that the trumpets are playing in (without a lack of support from other instruments such as low brass or strings). I'd try pairing additional instruments for the hits to give it more foundation.
Strings/Choir: Strings and choir seem to be lost completely at some points (example: 0:00 - 0:38). It'd be nice to pull up the strings/choir, and lower the brass.
Percussion: Unfortunately, I've never been much of a percussion writer, but I think that the timpani hits also get lost a bit. It may help to implement some snare, especially during parts where there is more action in the percussion.
Dynamics: I'm not sure what DAW (digital audio workstation) you are using, but it doesn't sound like there is much dynamic variation throughout the piece. If you have used a MIDI to arrange this, I'd recommend purging the preset dynamics/velocities, and then starting from scratch. Often times these MIDIs would contain artifacts that would totally screw with the dynamics, and it's just best to remove them.
Sampling: The trumpet sampling sounds a little unrealistic, but some of the string samples sound great. I think that no matter what samples you use, if you look into humanizing your samples with dynamic control, you'll find that you have a much better result. This YouTube video helped me gain better control over my samples, and I'd definitely recommend checking it out. (It's a tutorial for Logic Pro 9, but you may find some of the concepts helpful in creating more realistic sounds with your current samples.)
I hope I was able to help Nom! I look forward to seeing more of your work.