I disagree with your mechanic - you were doing exactly the right thing for your tranny changing it every 30k, and there is no reason to think changing it caused a problem. You were preventing ‘gunk’ from collecting in the first place! You’ve now gone 64k, if it was me I’d change it. But the fact you’ve been having rough shifting points to a transmission problem completely unrelated to your changing the fluid. And the problem may get worse, again not related to your changing the fluid.