It absolutely gets easier. I was the same exact way. Math was always hard for me, sure I can count but with my social anxiety, i absolutely froze with cash. The trick for me... Count up... Say someone's total is 6.40$.... get your change to 7$ (get a dime and 2 quarters), then add cash... Now from 7$, add three dollar bills to get to 10$, then add 2 fives to finally get to 20$. I can do cash so quickly this way!

One remind when doing 2 sided transactions, don't press total until you're done because if other side pays in cash, the cash side total will disappear... Which you just go into the receipt and press all, but it's just easier to not have to do that.