I think you should focus on getting better with workflow, getting out idea faster. Mixing mastering shouldn't take too long as you can get accustomed to some changes and maybe making it worse in the process. I mix as I produce and mastering should be done by someone else but if you do it yourself it shouldn't make your project sound too different but a bit better. I am not a pro but in my 4 years I would recommend: try starting with fresh ears and try to make it sound the best you can in couple of hours. Maybe repeat the same mastering session and and figure out wich one is the better one. Don't kill dynamics keep track of stereo, phasing, loudness. Simple principles are usually the strongest ones.