You sound exactly like me when I was a new nurse-- a whopping 1 year and 10 months ago. I thought I was losing my mind because I was exactly like you-- and as a person coming in with 2 previous careers, I had NEVER had the sorts of issues you describe-- esp memory stuff. But in nursing, there's so much information flying at you from so many different sources (MDs, patients, secretaries, care partners, other nurses, physical therapists, etc), you'd have to be a machine not to forget things. All this to say, unfortunately, what you're going thru is normal.
I totally agree with the nurse who uses different colored pens to write down important stuff; and find a mental system that works for you-- we're all different, so play around with it. I tend to give PMH 1st (if the ns doesn't already know the patient), then prns (because I'm always worried that I'll forget those), and then systems review (with any tests included, such as a CT) under the system that was scanned.
But you sound like a good, conscientious person who is concerned that you might have missed something-- that is a good thing!!! Keep at it; remember: you will make mistakes (which, as some nurses seem to forget, is part of the human condition), but you'll learn from them! And as your knowledge base increases & your time management skills improve, so will your confidence. By the way, somebody referenced the 5 year mark for competence-- it was Patricia Benner, a nurse theorist who wrote From Novice to Expert. She claims that it takes a minimum of 5 years to go from novice to expert-- and boy do I know it's true!! So, hang in there!