Water chemistry is a complex topic. Do you understand the water profile of your base water? If not, that is the first step. You might want to have it tested by Ward Labs.
The simplest answer:
Gypsum, calcium chloride, and dark roasted grains each result in lower pH, i.e., are acidic in the mash and the beer. When you are using all three, you might find the sharpness to be too powerful as you have experienced. For dark beers you may find it beneficial to skip these salt additions and instead perhaps add baking soda or pickling lime, as these salts will increase the pH to balance with the acidity from the dark grains.
I do not consider myself an expert on water chemistry, and to be perfectly honest I prefer not to spend much time focusing on all the specifics, but other helpful folks in this forum will certainly provide far more detail if you are interested, and soon, I'm confident of that.
Cheers and good luck.