Justin,
I commend you on creating such a large work at a young age, however I must agree with flint and Nikolas, the lack of variety in the middle and background planes is overwhelming (or rather underwhelming). Another rule that I learned long ago and continues to serve me well today: "Less is more". That doesn't mean that you can't craft a composition of the same length that you have. What it means is that you must be succinct in what you are trying to say. Without this you run the risk of losing the attention of your listeners, and that after all, is the very reason we write music that is to be performed in the first place.
Secondarily, you need to look at your choice in voicing many of the elements within the piece. For example, mm. 128 in the first movement, the descending eighth notes in the trumpet and trombone.
First, the trombone. The voicing of thirds particularly below Bb1 is likely to be muddy particularly when playing marcato. Sometimes this is a desired effect in more avant garde music but we would all agree your music does not fall into that category. Ironically, since you are a trombonist it would seem that you should know this better than most. When voicing any instrument in this range (tonally), a good understanding of the overtone series is paramount, particularly in the brass. Study examples of Holst, Shostakovich, Williams, etc.
Next, the trumpet. In general, your use of the trumpet could be better served with say, the horn, or even trombone. In many of your passages you take the trumpet down to their written G2. Functionally, it's playable but there doesn't seem to be any real context for it.
I hope these comments were helpful.
-Ray