It takes a lot of time to get comfortable. When you first step into a casino and try to count, your heart rate will be so high you won't be able to think straight. You'll have tunnel vision, sweat, you'll be panicking about the pit bosses. Same things happen to soldiers and cops, which is why they try to do a lot of practice with live ammo before they get into a bad situation.
Your heart rate will be so high you literally won't be able to play well.
But don't worry, after awhile, you get used to it, and your heart rate doesn't spike as much, and you'll be able to focus. You may have to consciously control it for awhile: breathe deeply, remember to count, etc.
I still have to take conscious steps to control my heart rate when I have a huge bet out and start splitting.
Practice helps a TON in this area. I had played hundreds of thousands of hands of BS on a computer before I ever played in a casino. This helped me avoid basic strategy errors. I'd also recommend a lot more counting practice. Maybe have a friend deal to you.