For your first- Bitterness in beer typically comes from hops. What was your boil addition schedule on the beer? Also when it comes to beer involving whiskey it typically gets much better and flavors mellow out with time. Many kits involving bourbon barrel brewing ask for 6-month conditioning times. There may be some off flavors that you are picking up now that will be completely gone a few months from now, problem is leaving the beer that long without drinking it!
As for additional ingredients, there are tons of different things you can add to the beer to change the flavor, and many ways you can go about using it. Put it into the boil, put it into the secondary, put it in at bottling are the main ways it is done, and they will alter the flavors greatly. As you are a new brewer (as am I as well) I would suggest looking at some flavor extracts and do some reading about the different times to introduce them to determine what is best for the flavors you want to achieve. You can add real fruits or berries to the beer but they usually don't impart strong flavors into the beer. Another common addition is coffee, and there are tons of ways to go about adding it as well.