Extreme bitterness could be due to tannin extraction from the steeping grains. Tannin extraction is more related to pH than temperature. Steeping grains in large volumes of water can increase the likely hood that that grains will not buffer the volume of water to reduce the pH below 6.0.
Another way to produce a very bitter beer is to boil prehopped LME in a kit. Give us some details on the last beer you brewed that was to bitter to drink.
Flars, I'm sorry but I can't provide you with any details, This was probably 5 years ago. I brewed a Bock and an Blonde Ale, Both were very bitter.
I know from one person, One of the beers I made, he tasted Salt, At that time I had a Water Softener in, I don't anymore, I didn't taste it probably cause I was used to it. And it was extremely bitter.
Then the blonde ale I brewed, it was very very bitter as well, no water softener this time I do know.
My Normal brew was 2 Gallons of water, Heat it to the temp, steep the speciality graisn for 30 minutes. Then bring to a boil. Pull off stove stir in LME so it wouldn't scorch. Then put on stove again, bring to boil and start putting in hops at the times needed.
Now I do recall that none of the LME I had was previously hopped. It was a Bock Don't remember the rest of the name, and it was a blonde ale I brewed.
Right now, I haven't brewed them yet (Hoping they are still good) is a Germen Oktoberfest and an Oatmeal Stout. Oatmeal stout Kit I got is a All Grain.
Both has been sitting for probably 4 years maybe 5.