Can you post a link to one of the songs you exported? There are a multitude of possible reasons why your rough mixes are not good.
Here are things to think about:
1. The most important part of producing good mixes is having a good monitor setup and listening environment. Think about the size of your room, monitor placement, room treatment, listening position, etc. There are tons of guides for setting up an optimal mix environment.
2. The second most important part of producing good mixes is starting with well-recorded source material.
3. Third is gain staging. Making sure you have optimal signal flow through out your audio chain is key to producing good recordings.
4. Do not over use EQ, Compression, Reverb, etc. Your mix should sound good dry before you enhance it with EQ, Compression, EFX, etc.
5. Use reference tracks as a comparison. Listen to tracks by other artists that you like through your monitoring system in your mix environment. Get to know what a good recording sounds like in your mix environment.
6. Listen to your mixes in multiple different locations: mix environment, living room, bathroom, car, headphones, etc. Knowing that your mix sounds good in all of these places is important to understanding how to produce good mixes.
7. Once you develop a good mix workflow stick to it. Save a mix template, save you plug presets, and start there next time.
There are lots of other tips, but those are the tips that live by.