You need to be incrementally exposed to an increasing number of
marimbas in a reassuring way that raises your comfort level, then
do the same with xylophones while taking prescription anti-anxiety
medication under continuous monitoring by qualified professionals.
I could not help you with that unless you are willing to pay the
exorbitant fees and expenses that this would entail, none of which
would be covered by your insurance. I suggest you just avoid
xylophones, which are rarely encountered in everyday life anyway,
and just focus on the aspects of life you enjoy.