When you have a boss that doesn't train you properly, yet still expects you to know stuff about a field you've never been in, coupled with employees who don't like the fact they're taking orders from someone younger than themselves...
It all adds up to you just feeling like the most useless, incompetent waste of space ever. My low point was staying behind by myself after the shop was shut, just trying to get as much work done as possible so I didn't feel so useless. Just ended up sinking to my knees and sobbing.
The impact that whole ordeal had on my self confidence was huge. It has taken me quite a few months and successful placements to gain the perspective that it wasn't my fault and I am, in fact, a capable person who people should hire.