I guess the first thing to do would be to focus on mixing and stop thinking about them as one "mixing/mastering" entity. They're separate processess, which admittedly have some overlap, but basically your mixes should be 95% there before hitting mastering - otherwise it's gonna be a "polishing a turd" situation. Mixing is usually done in a DAW, and all DAWs offer at least basic (and nowadays perfectly capable) tools for making competent mixes. So chances are you already have what you need; now you just need to learn how to use them.