"The addition of “phantom aromas,” such as vanilla, berry, citrus, bacon or even cheese, can distract the brain from acknowledging a bitter to taste." "Other additives can mask or “mitigate a bitter taste.” Lactisole, for example, made from carboxylic acid salt derived from Columbian coffee, can negate sweet taste. An allosteric modulator can change a food or ingredient’s protein structure reducing the salty, sweet or bitter signal to the brain." src -http://www.novotaste.com/content/bitter-blockers
Substitute some of the supplemental caffeine with theanine, and add only Xylitol (possibly steviol but to me the taste is disgusting). Don't forget some glucose-based sugars for consistency (it considerably affects aftertaste). Add a pinch of potassium-balanced salt. Lemon juice helps the bitterness too if that is an option for you.
