Let me tell you what I did many many years ago, and it may work for you. Find a professional recording studio in your area. Send them an email introducing yourself and start a relationship. Tell them what you want to accomplish, your weak areas as well as your strong areas concerning mixing and mastering. Most people love to teach what they know. Ask them if you can sit in on a couple of secession while volunteering to bring the coffee and donuts.
Keep your mouth closed and your eyes and ears open. Take notes and save any questions till the very end of the secession. Don’t overwhelm them with questions. Take your time and just listen to what they are saying. Google their answers and do your own research. Mix your audio and send it to them to master it. Also include the raw files as well. List all your equipment as well as the software and plugin you used. (Most Professional Studios Request/Require This Information).
Ask them to include any suggestions on your mixing abilities. You may not be as bad at mixing as you think and they should be happy to do this. Tell them you will use their service exclusively for your audio. Most people who mix their own music will send it off to get mastered. I do not want to start a debate on this but there are many reasons they do this. Find studios that have “Open House” and attend everyone you can. This is what I did and it changed my whole concept on mixing and mastering audio.
This forum is for discussing recording vocals as well as instruments.