Well as far as samples go; I use Sampletank 3 SE, which has pretty high quality samples. But I think it has less to do with samples and more to do with how the samples are sequenced. You could have the highest quality guitar samples or string samples; but if you don't know how to realistically program them, they won't sound good.
It's not either or; it's both. You do want to know how to get the most out of your libraries, but you can also only take cheap libraries so far. As far as orchestra libraries go, Sampletank 3 SE is more on the entry level. There's just a huge difference between a 6.5GB orchestra library, and a library that requires 26GB for the violins alone—or between a library that costs $99 for the full orchestra, and a library that costs $399 for the violins alone, to put it in another perspective. If you want realism, the sooner you start to use such high level libraries, the sooner you get to practice using them. They also tend to sound better out of the box compared to cheaper alternatives, so that's another benefit. :)
As for my mixing; what could be improved? The EQ? The panning? I would like to know that as well.
There isn't much to add to what CCH Audio said. Focus on the fundamentals: levels, pan, EQ, compression, reverb. This is not where a good mix starts, though. Understand this production flow:
Composition -> Orchestration -> Recording -> Mixing -> Mastering
A good mix begins with the composition itself. The better your composition, orchestration, and recording; the less you will need to fix in the mix. You may be aware of that, but I think it's always good to have a reminder.
You also used like a pan effect on one or two tracks that I thought distracted more than anything else.
Again for emphasis: Music is an emotional experience. Composition is drawing the emotional curve (tension, release, etc.) of your song. Orchestration is enhancing the emotional curve with instrument choices, adding harmonies, and so on. Recording/performing is interpreting the emotional curve and bringing it all to life. Mixing is making the emotional curve perfectly clear and possibly enhancing it with some effects. Mastering is the last check to see if the emotional curve is perfectly clear and everything is as it should be.
This is probably the best advice I can give. Composition in this context I consider more or less what you would play if you played your song with one hand on the piano (in most cases that's the melody).
What we are both saying is that your orchestration is not 100% doing that (in some songs taking away from it even), your recording/performance can be improved to bring it better to life—with better programming and more realistic samples (perhaps live recording?)—and you can make everything still more clear in your mix.
When you start to really focus on the emotional curve in your music, I think you can really see yourself where you can improve this or improve that. You definitely have far more potential than you are currently showing. You can easily charge the kind of rates you're looking for, but you have a lot more potential than that.