"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.

Leaving any amount of fructose in is really not worth it, since outside starvation it will not be used up rapidly and will definitely proceed to deliver permanent metabolic damage including diabetes.