Hey there,
I'm sorry to hear you're feeling stressed, and I totally understand where you're coming from - but please don't worry. There is nothing you can do to alter the outcome now.
When dissertations are marked, it is hugely unlikely that lecturers will deduct more than 1 or 2 marks for one simple mistake. So if you have done a good job overall - followed the scientific method, critically evaluated your sources and analysed your own evidence against these, explained your findings and proposed what can be done next etc - one or two marks deducted will not prevent you from achieving your First or 2:1. If you are still worried, then perhaps contact your supervisor and explain to them what happened. I'm sure they will be able to reassure you that one mistake will not cost you your degree.
I really hope this helps - keep your head up, try not to worry - and best of luck 🙂
Holly
University of Bath