in my opinion

mastering should not make or break a mix. IF it sounds bad before mastering , it will sound bad after mastering. I would look at other elements of yoru mix first hand before messing with mastering. SOunds like an eq problem you are having.

Me personannaly i have'nt really found any shortcuts for getting great eqs. There are tons of guidelines but i think it just takes time to develop your ears.

I usually listen to music very closely for one hour a day and try to get good mix sound thru osmosis!!