I would try setting this wine aside, in the carboy, and allowing it to bulk age for a good long while. Bulk aging allows some of the bitter components to drop out, given enough time. Another thing that might help--if it's still somewhat bitter after bulk aging-- is to try some glycerin or vanilla in it. 1 tsp of glycerin per gallon. 1/2 oz of vaniila extract per gallon. Vanilla extract is much better if you make it yourself, rather than using store bought vanilla.
Optionally, you could also try some Casein in it after the bulk aging.