Bitterness is to do with controlling the yield of solubles in your extraction. Coffee grinds have about 30% water soluble constituents. Of this, we target around 20% extraction for a good tasting cup.
This is how the different solubles extract in coffee brewing, regardless of brew method used:
If you have a grinder handy, you can start with slightly coarser grounds to fix this problem. Start with small incremental steps.
If you are using pre-ground coffee, then the three options are to change:
a) the amount of coffee
OR
b)the amount of water
OR
c) the brew time (or extraction duration)
This explains the relationship between the three:
A good starting point for immersion methods like cold brew is coffee to water ratios between 1:13.5 to 1:14.5 i.e. 1 gm of coffee for every 13.5 gm of water and so on. Start from there and tweak it up or down till you get your desired taste.
Our suggestion is to change one variable at a time - either grind size or coffee or water, to make this a manageable problem. Since this is cold brew, best to leave the time variable fixed as it could take many days to determine the ideal value.