The gasket insert thing for the mixing valve can be accessed through the front. Just take off the handle. There's usually a screw on the front or a set screw on the bottom.
Get that off and you'll see the inside. Make sure you turn off the water for the next part. There should be a pin that holds the gasket insert in place. Pull the pin. Then get some channel locks, grab on to the front of the gasket, and pull it out. You may have to pull it out pretty hard.
Go to hardware store and buy a replacement. Then push it back in, put the pin back in, and put the handle back on.
But I'm not sure that will solve the problem. Haven't run into your particular problem before, but it seems plausible!
Some things to look for: is your mixing valve/handle old style or new? As in, can you control the temperature and pressure? Or just the temperature?
If you can control both, it's a pretty cheap piece. But when you put it back in, check that hot and cold are still appropriate. If they aren't, you should take the gasket back out and flip it 180.
If you have the kind that only changes temperature, the piece will be a bit pricier, but there should only be one way the gasket goes back in.
Good luck!
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