I would recommend using Determination. Mythic Rhythm snap--shotting really isn't necessary unless you are pushing up higher.
Its really your gear more than likely. You have all the right pieces, but kinda poo rolls on things such as your F/R. (Nice ammy BTW, too bad on the passive).
Just between your two rings and hands you are missing out on 150% CHD , your fist of AZ is only 85%, and your helmet has no Crit.
Also are you using mythic Rhythm correctly, or just using that talent because you saw it in a build? If you are just 1,2,3 punching then SSS, that talent is going to waste basically.
You should be able to push 60 without too much better gear... but you have a LOT of CHD, CHC and CDR missing from your gear so keep on farmin!