How loud are we talking about exactly? I try to do most of the critical work around 85dB SPL which is the level we hear relatively flat at, but even that is pretty 'loud' to me for long periods so I turn it down in between.

Taking frequent breaks helps, get up and do a lap around the house in silence. Continually drink water or tea so that you are forced to get up and go to the bathroom frequently (plus it's good for you!).

A lot of things can play into ear fatigue though, not just the sheer volume. Your monitors will play a huge part, if they tend to be fairly mid-forward and present that is going to be more fatiguing to a lot of people. And similarly, how well you're actually mixing will have an impact too. If your mix is full of overlapping elements fighting for space in the midrange and being squashed by compressors and limiters, that right there is going to get very fatiguing very quickly, and not just to you but also to the listener at the end of it... another reason to not participate to heavily in the loudness wars.

And this is a bit yucky but it's a good idea to clean your ears often.

yeah the key is to take breaks. often.

Download an SPL meter for your phone. It won't be perfect but it will give you a good idea of how loud the music you're listening to really is.

The trick ive used, is to always set my audio interface volume knob to the same position. That way I'll get used to that volume and know how shit sounds when the knob is set to my default position. You can take a marker and mark a little spot so you'll remember the volume position.

Ofc this requires some listening and judging, it takes some time to find the right volume.

If u dont have an interface just do the same with windows mixer or whatever your using.

This has helped me to get used to lower volumes.

I rarely produce with my headphones on, I'll do most of the stuff on my monitors and then put on the headphones to make the final touch, even then I'll don'y use them 2 hours max. I listen to things a lot quieter than most people do, so I don't get much ear fatigue.

it takes some willpower but keeping your volume at a safe level is very crucial. of course it's fun listening to it if it's blasting you in the face, but that often translates to a sub optimal mix. i know i get a better mix 100% of the time if the volume isn't super loud(this is partly due to ear fatigue and partly due to the fact that everything sounds better loud). i turn it up every once and a while, but usually only every hour or so for one play through.

Nothing new from what others have said.

But, maybe you're just to attached to the work that you're doing.

It might be beneficial to take a step back and just call it a day when you feel that you need to push yourself. Don't get me wrong, it's ok to do that every now and again, but you have to be smart about knowing when you can push yourself and when you just need to let it rest.

Quite the difficult balancing act to feel.