Well, as far as I know, once its bitter, its bitter. If it's mildly bitter, you can always use it as a substitute for greens that are meant to be somewhat bitter, such as frisee or escarole. But, at some point, its just inedible for humans, and you can compost it, feed it to poultry or rabbits, etc.

Really, in the heat of summer, its hard to grow non-bitter lettuce. Personally, I find it better to just grow lettuce in the cooler periods, and stick with things like chard, orach, malabar, and amaranth for the hot weather periods. But, if you want to try, try growing it in afternoon shade in really rich, moist soil, and harvest it as "baby" lettuce.

You can make what we call "killed lettuce" with it. Throw it in a pan of hot bacon grease long enough to wilt it. It's not healthy but it isn't bitter anymore!

Agreed there is no cure. Too late to replant until it is time for a fall crop.

Dan

If is mildly bitter you can make a sweet vinagrette such as balsamic+rasp or strawberry jam+dijon mustard+S and P,yum with some locally grown strawberries...enjoy.

Vinagrette with some shallots in it will nicely complement the bitter greens including bitter lettuce. In fact, it works better with bitter greens than bland tasting greens imho.