Hi Jo,
You are doing very, very well - everyone has down days, when they are tearful, frustrated and convinced it's all been a terrible mistake - if you don't have one occasionally, you should be worried! I threw wobblers several times despite a very easy recovery.
I only went up stairs at night and down in the morning, like Roy. One morning during the third week I felt a bit strange halfway down the stairs, wasn't quite sure why, thought I'd got there a bit quickly and realised I'd been going down them normally (each foot on a separate step)! So that came without even thinking about it. You will find that your confidence will build too and you'll soon not be worried by your dodgy stairs.
It sounds like you have a lovely, quirky old house by the way. Devon is gorgeous, how lovely to live there.
You are very lucky with the sleeping. To sleep well so soon is wonderful and will probably speed up your healing as well. So relax, you are well on the way to recovery!