As Martin said, mixing and mastering is an art and Cubase does have built in plugins to help with mixing ( Compressors, EQ’s, etc.) but there is literally thousands of third party plugins to help with the process. You didn’t state if you have applied any of these to your tracks to get it sounding better before exporting. There is a section in the forum for VST’s and other plugin related issues, maybe try there for that sort of info…or google and you tube it! While I’m here though, Izotope have great plugins to help with mixing and mastering (just my opinion, as it’s subjective) they are called Neutron (mixing) and Ozone (mastering) give the free downloads a try and learn and practice and practice and practice!