Why make the same mistakes twice? I would write down the major mistakes I've made (like not showing up when I was supposed to take the other person's shift) and a way to make sure that doesn't happen again. Writing down your schedule (and then referring to it) might be a good start for that one?

Make yourself a habit to write things down and then read what you have written so you do not forget things. Everyone would forget things if they weren't written down, that's why doctor's give us little cards for when our next appointment is and why there are calendars and day books. If you have a cell phone, learn to use the features on it that might help you, including alarms, etc. Practice focusing; it is not automatic for anyone, we all have to learn to do it. Yes, it comes easier to some than others but everyone has to learn to do it and practice it before they can be good at it.