Recipe might help. Perceived bitterness could be too much hops, too much roasted grains, or something else.
First, the bitterness will probably mellow with age.
Second, assuming you haven't bottled it yet, if you really think there's a problem you could boil some lactose in water at bottling time, and add a little at a time to the bottling bucket (then taste) until you think it's "right". This won't necessarily "fix" it, though. Depends why it's too bitter in the first place.