If you're using what is termed "chili powder" (as opposed to diced or ground dried chilis), it could be either too old, or you've used too much. Regular Chili powder is generally used only for flavoring and mild heat. The real heat comes from ingredients such as Red Pepper Flakes, Cayenne Pepper or other ground pepper mixtures such as Jalapenos, Habaneros or Scotch Bonnets. Tabasco or Louisiana Hot Sauce (my favorite) are also popular to increase the heat - all without using too much Chili Powder.
Using White onions in stead of yellow sweets or Vidalia, can sometimes impart that bitterness as well. Try adding a teaspoon or so of sugar to a small sample bowl (not the whole thing) and see if that helps.
"Food is our common ground, a universal experience." - James Beard