Hi! Joseph is spot on.
All the online automated mastering services are like asking Siri to baby sit. More damage than help.
Mastering is the the thing you should attempt after years of getting your mixes just right. You need a reference quality monitoring set up, a room that is very flat to very low frequencies. My room goes smoothly down to 17 cycles and that is extremely rare.
As a Music Producer(Just nominated for a 2020 Grammy) who does master occasionally, I have mastered 500 albums and singles. BUT my pro mastering buddies have done 5000+. Perspective and subtlety are crucial. If you just need to apply some leveling help to your mixes then many programs can help (iZotope's Ozone among them.) Having said that I watch college students mangle their work with it all the time. I would start with learning to reference your mixes to tracks that you love and turn them down 8-15 dB while mixing so you compare them at mixing levels. Then AFTER you love the mix attempt some level and limiting touches post mixing. Feel free to contact me for Production or Mastering Lessons or if need be Mixing or Mastering. If you mixes aren't awesome I will push you to work on your mixing first.
All my best to you, Daniel