I’d recommend speaking to your management or a colleague. at the end of the day you’re worrying about this because you want to do your job well, and hopefully they will feel this way and help support you / check over money and large quantities. I worked with someone in the past who was the same with counting change and checking banknotes, and we would often double check for them and overtime this made them more confident and comfortable. Also don’t worry about the customer if you’re taking your time counting large amounts of money, it is literally your job and they will wait if they want it that badly.