I'm a little late in replying, but I thought maybe I could offer a bit of advice possibly! I just got done with a clinical rotation where I had an instructor that would drill us on tons of stuff with our patients and as much as I hated it, it was really beneficial to my learning. I froze up the first few times and I really felt like an idiot! But I learned what I needed to know, or rather, what SHE wanted me to know.
Make sure you have all current labs out in front of you on a sheet of paper for any kind of meds you may be giving. Make sure you know all nursing implications for you meds (ie...do they need to be taken with meals? Is the diuretic K+ wasting/sparing?) Know the reasons why you're giving all meds and the rate at which you need to be giving all of them because as you know (grr lol) it's not always written on the MAR. Write this stuff down! Even if he/she yells at you for looking, at least you're aware and you're not just wandering in the dark. Plus you're more apt to remember this stuff if you write it down, yanno? Just be sure to keep updated on vitals/O2 sats if the pt is COPD (I'm not trying to insult your intelligence, I swear lol...I make this mistake CONSTANTLY lol).
I hope this helps...if this has nothing to do with the mistakes you're making then I'm sorry! This is just the kind of stuff my instructor drilled us on. I know when we were giving a diuretic we had to know that K+ level was and if we had to look it up we got in big time trouble.
Good luck hun, try not to be so hard on yourself!