There are lots of potential causes here:

You have excessive expectations of yourself - has your manager said anything?
You are rather anxious and this affects your concentration
Fiddly intricate work such as spreadsheets is not really your thing? You do say you are a bit of an ideas person.
You might need some more training - perhaps making a working copy of a spreadsheet to work on before updating the real thing?
Perhaps others need spreadsheet training - should not crucial rows and columns be locked?

Just a few thoughts - I doubt there is anything much wrong with you, but you might wish to explore why you feel uncomfortable.