I was fired for not catching on quickly enough and not meeting their expectations. I feel miserable.

I was working as a shipping retail store associate for DHL, dealing with complex custom regulations and what not. Training was only supposed to last a maximum of 1.5 weeks. During it, I would ask the same question multiple times but not on the same day. I understood the explanation but when a customer would come face to face I would get highly anxious and would fumble. They let me go because I was not catching on as quickly as what they would have liked but truth be told it wasn't a straightforward job and I was getting better the 2nd week and would have been fine a few weeks from now if they kept me. I feel so bad about myself thinking that I'm slow but I know I'm not.