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I have always had a problem with "overexposure" of the mix. I don't think it was a problem caused by listening sessions
(monitors, headphones).
I am using SPAN to preview the frequency characteristics of the mix. Usually, I try to set all the instruments in the SPAN
to "flat", but this doesn't always work ...
Do you have any good ways to get rid of this problem?

Take breaks to reset your ears and have some mix references. There are also some mixing tools that analyse your mix like iZotope's Tonal Balance. Mastering the Mix have several plugins but they are not a silver bullet.