Mastering, unfortunately is more than just popping in some plugs that other people tell you that you should use.
It is a process that I believe starts at the tracking stages.
2 people can take the same project and using the same plugs get 2 totally different sounding mixes.
It is a process that takes time to learn.
To my ears on this system the mix sounds balanced but lacking that quality that takes it to the next level. Not too shabby though for an overall good job. The finer points will come as you continue to mix and learn not only what to use and where, but what the plugs are doing and why or why not you would want or need to use them in a given situation.
The vocals over all on the various songs sound a bit muffled. On the songs I mix, I do not use (generally) compression on the main bus just because I have them.... I use compression, limiting, or EQ, only if I need it.
I would recommend getting and reading a few books on the subject of mixing and mastering in home studios. Mike Senior's book is a good one to read and study.
You did not state what monitors you use nor did you mention the tools. The listening environment really matters to getting a good mix that holds up across many playback systems.
For mastering made easier......Many folks here use Ozone or another of the "mastering suites" available. They are not a cure all, and can easily screw up a good mix if misapplied, but they can provide good starting points.
In addition, on the Ozone web site ...go there and find the MASTERING GUIDES and download them. They are free and are packed with really good info to help you understand the what, where, when, why of using the tools.