Several reasons for a bitter taste. If you pressed the cherries really hard you may have introduced what are called phenolic materials into the juice. Excess tannin, or succinic acid can create a sour taste. If your wine oxydised it can give a bitter taste as well.
Were your cherries fresh picked? If they were young and not sweet enough you may have excess tannin. That is the case with grapes, I'm sure cherries are the same. Were they fermented with seeds and stems? This would also account for extra tannins in the juice.
Was the cherry wine cleared or finned. That will remove some bitterness as it would remove some tannin and other compounds causing a bitter or astringent taste. Or possible it just needs to be sweetened.
I just looked this up you could add a protein called casein. If you truly have excess tannin etc. then casein would remove the bitterness. You would only need a small amount. Geletin would do the same. I never used either.