Mental health at an all time low- resits

You might need to, dependent on the status of the unit and how it relates to the year. If it's a non-core unit, then you'd usually be ok with a summer resit.

What's a non-core unit?

A unit that is not classed as fundamental to the year's progress. My dept has 4 core units in years 1 and 2 but none in year 3. Failing those is lots more problematic than failing elective units. But you'd know this if it was the case.
I misread your initial post - if it's your first fail then 99.9% of the time you will just do a summer resit.

Oh phew, 😭 I got so scared for a second. Yes it's my first fail, ever actually and I dont think it's hit me yet, I still don't know how to deal with it. I think it's because of burn out. People always spoke about burn out but I never thought it would affect me, because in my head I thought I would never let myself get to that stage, but here we go
I'll just have to give it my all in the summer, I dont know how to stop thinking about the exam till then

The key thing here is to have a bit of perspective on it and to be kind to yourself, honestly. It is easy for me to say and a bit harder for you to do, but it's nevertheless the truth.
You have some time between now and then to reset yourself and try to regain a bit of confidence and a bit of a head of steam. The work is of course important, but make sure that you have some time for and to yourself. Switching off from university is very important!

That's true, thanks for taking the time to reply