matt sez: "I think everything sounds fine. Until I compare it to what other recordings sound like... "

And in fact, right there, you have hit on a key technique. It's called "reference."
The theory behind it is that your ears get accustomed to whatever you are listening to, and it sounds okay while you're listening to it. Even if it's not.

The reference trick is to copy a song off of a CD you want to sound like, and import it into your mix project as a stereo track. Once you have your rough mix done (with the reference track muted, obviously....) mute your stuff and un-mute the professional track for comparison. Listen critically, shifting from one to the other, and see if you can identify a specific difference. Less low-midrange clutter in the pro mix, for a common example. More air in the vocals. Mute the pro track and go back to your tracks, and see if you can adjust to get closer to the sound you like.

Back and forth, back and forth... patience, grasshoppa... eventually you learn something. And as Bernard said above, "It takes years of practice, and there are no shortcuts."

And mind you, this is only one trick. It can improve your mix skills but it can't make a fundamentally crappy recording gleam. So things like mic placement, room conditions, instrument and equipment quality, all need attention from start to finish.

The journey is rewarding though. I've only picked up this reference mix trick recently, and I've been amazed at what it's helped me with. And there are tons of websites devoted to home recording tips (some rubbish, some very good, and only you can be the judge of what helps you). No reason to struggle without help.

Cheers
-Glenn