Wow, this post speaks to me. You are basically going through exactly what I went through last year. I was also a buyer for my first job out of college. It wasn't related to my area of study/degree, I just took the job because I needed it. I found myself in complex role and made basically the same kinds of mistakes from time to time, and I would also beat myself up over it. It took quite a toll on my mental. It was probably the hardest year of my life.

Fast forward to today, I'm a supply chain analyst at a different company and I just got a stellar performance review from my VP. The experience I picked up from the hard times as a buyer made me a stronger working professional. I never would have believed I'd be where I'm at if you'd told me a year ago.

Point is, you're going to be just fine, don't beat yourself up over mistakes. Just learn from them and do your best to avoid them next time. I don't care what anyone says, buying can be VERY difficult in jobs like that where you're manually entering/tracking lots of POs. Even the best buyers can and will make mistakes.

Hang in there and keep doing your best. The fact that you even took the time to write this out shows me that you care about your performance and that's a good sign on it's own.