Give it a good six months. I felt like I made so many errors when I started. The patient deteriorated, and it was all my fault? Of course not, but I could not see why. I was so tired and stressed, the whole anatomy and physiology of intensive care was beyond me.
But after a while it did make sense. It did 'click' and I got good feedback from my mentor. It did get better. But the first few months were really tough, and I didn't think I was any good at all.
12 years later, still in ICU, clinical nurse specialist. Some days it is crap, other days I love it. :)