yep, bitterness will fade with time. but don't judge it too quickly. give it time to carb up in the bottle and i bet it will taste fine.
were these whole hops? there's no need to grind them up, you can just toss them in as is. i wouldn't expect much bitterness to come from these hops if you added them toward the end of the boil. you could get some, but if they didn't boil for very long, i doubt that is what caused the bitterness you describe. did you also add hops at the beginning of the boil? is it hopped extract or something?