how to manage stress/anxiety before assessment centres/super days?

i don't know what it is, i don't think i suffer from anxiety in other aspects of my life - i've always done well in interviews, exams, presentations, etc. 

but for some reason, the days before an interview stress me out like crazy. i'm a university graduate and i've done a couple of off-cycle investment banking internships (so it's not like i'm new to recruiting, and have also done demanding ACs before) and have a solid CV. i'm now looking for an off-cycle i can convert to a FT role. i've been invited to an AC soon for a relatively good firm, and i've not been able to de-stress for even a moment. 

i've been preparing solidly for the AC, but of course nothing feels like enough, and i think i'm just ultimately struggling with the randomness of it (and also knowing i probably won't sleep at all the night before the AC). 

i know i could perform amazingly in it and not get an offer. i also know i could feel like i've performed badly and they still see something in me. i thought having gone through recruiting a few times before would ease me and make me realise how meaningless stressing over ACs are, but in reality i'm sat here wondering how i even got my past internships to begin with or coped with how demanding IB recruiting is. 

would really appreciate any advice or perspective from others on, essentially, how to chill the F out. when i got the AC invite, i was indifferent and quite healthy about it, i.e. i acknowledged that it doesn't matter how i do because beyond a certain point, it's out of my control (and influenced by cultural fit, politics, etc.) + opportunities are abundant, but the more time i invest into AC prep, the more anxiety-inducing it feels tbh.