A couple of points that I've seen and I want to put them all into one post.
First;
GIGO - garbage in, garbage out. Sloppy playing, out of time, out of tune...you hear any of that - scrap, start again.
Second;
What you hear is misleading if;
-the room isn't treated
-the monitors don't exist
-you're listening at too loud a volume
-there's no monitor correction software, technique
Third;
-the only rules are, there are no rules.
-compression don't fix everything
-eq don't fix everything
-effects don't fix...well, wait...no, they don't.
-bussing is a technique, not a fix.
-panning don't fix everythign
Fourth;
-the sound spectrum must be complementary to the instruments. While it would seperate the mix a little, it's not smart to put the kick wide-right. So judgement is your best decision. Don't just pan. Know why you're panning. What is the effect you're going for? Visualize the sound to create an image, or a feel, or a sense then work the controls to try to make that happen.
To me, it sounds like what you're struggling with is that you don't know what you want, you just want it to sound better or tight. The only way you're going to be able to isolate what doesn't sound right is through failure and experience. There's no magic. Writing and recording are very different from mixing and mastering.
The writer loves his poems, the recording artists is overjoyed with hearing his work. The mixer thinks it all sucks until it doesn't. The master hates what the mixer did until it's perfect.
These attitudes are polar to each other. I know that is what I struggle with the most.
One rule - make small changes. Limit the number of them as well. Don't repaint the entire room because of one wrong color spill...
keep at it. the CW community is here to help. That's what I love about these guys.