Wow! It's as though you climbed into my brain to write this. A huge problem with me. I was especially blessed when you shared your experiences with not feeling like your mix was adequate for your clients.... yet...they were totally satisfied with it. I have experienced the very same thing as I wrestled with whether to believe them or not when they raved over it. I know it isn't perfect...but in the end they were happy and that's what counts. This is a great article. Thanks for sharing and I look forward to future posts.
I have two steps I go through to determine when enough is enough:
1. I listen to the song on my monitors, open back headphones, our house stereo and then in my vehicle. If I'm satisfied and can't come up with any needed corrections I go to step 2.
2. I have three friend who's ears I trust. One is a home recording artist himself. The second is a real audio nut with over 2,000 CDs and I don't know how much vinyl. The third (don't laugh) is our band drummer. They almost always pick up on something I missed.
Once those tests are passed it's time to press save and move on to the next project.