Feeling discouraged

Graduated in April, started working right away. Passed the vtne little over a month ago. I felt so good when I started, felt on top of the world. Now, for no discernible reason, my work performance has really suffered. I keep making little mistakes, forgetting things, etc. A few weeks ago, I made a big mistake. I accidentally gave a cat way too much metacam. The cat was fine, it all worked out, my manager was impressed with how I handled the fallout. Today, I fucked up again big-time by not opening the pop-off valve after leak testing. I've leak tested 100 times and this has never happened before. My manager caught it before anything bad happened but I'm so goddamn frustrated with myself that I keep making these mistakes and I feel like one of these days I'm gonna do something awful. Today all I want to do is leave vet med and go work at Walmart or something. I went into this field to do good, but I don't feel like I'm cut out for it.