want to quit so bad=(

well i am on a med sug unit and this is my last day on this floor till i go to my floor ccu step down. Well i made another mistake yesterday! I forgot to check a ptt for a pt on a heprin drip and then had to do a stat ptt and reajust the rate from that hours later! I really dont see how i can do anything better. Its so obvious that that should have been a priority. But i have no reason why i forgot. Maybe i am not cut out to be nurse the only reason i have not given notice is i feel it would be should a horriable feeling to fail at this. But it made me panic about when i go to the ccu step down. If i made this silly mistake, what about the ccu step down where i could kill some one with a error!!!!!!!!!!!!! ugh i feel horriable should i just quit now? please help