Hey, I was in this exact situation last week so maybe I can offer my perspective!
I accepted an AC offer back in December for a date in Feb (through a fast track scheme which was why it was so early.) I've been really fortunate to get quite a few AC's/vac scheme offers since, and realised that the vac scheme dates of one of the firms conflicted with those of the firm with the Feb AC.
I initially accepted as I considered both top choices, and thought, if I were to get an offer, it would help me to make the decision between them. However, I decided to pull out around a couple of weeks before. I decided that the first firm would likely be my preference, and, even if it weren't, I was still overjoyed with that vac scheme offer I had. More so, though, I couldn't imagine putting myself through another AC at that point in time. I completely understand that I'm speaking from a place of privilege (having other offers) but nonetheless, the process is arduous, and feeling as though you are constantly 'on show' can take quite a toll. There comes a point that you may just need to refocus your energy on other things, and I made the choice in enough time that hopefully, another candidate was offered that space.
It feels like such an honour to be selected for an AC, and I felt terrible at first for 'letting an opportunity go.' You do, however, need to look after yourself - you are clearly an incredible candidate if you have done other AC's! If you think you can, go for it, but if not, there is no shame in pulling out (it doesn't mean you aren't resilient.) At the risk of sounding harsh, if the prospect is bringing up these types of emotions (beyond regular nerves) you are unlikely to perform to the best of your ability even if you do push ahead.
I'm not a member of grad rec, but I can't imagine you would be barred from ever applying again. I simply thanked the firm for considering me and reiterated how much I had enjoyed the fast track scheme, but that I, unfortunately, would need to pull out and they wished me well! I think it happens quite often (for a multitude of reasons, not just conflicting offers.)
Hope that helps!
