There's five words in your post that inadvertently give the game away for me.
given you relax first, ofc
When I was still regularly consenting to anal (before I got my feminist consciousness and learned to stand up for my own comfort, safety and pleasure), I found it almost impossible to achieve the necessary relaxation of the rectum/sphincter without some outside agent. Which was mostly poppers, E and/or booze, for me, this was decades ago and I'm sure drugs have changed now, MDMA and Ket have probably replaced E. (Ket especially because it would relax you and also enable you to dissociate.) Also of course lots and lots of anal lube. LOADS of anal lube! Then towels, warm water, vaseline, codeine in the days following for the pain...
The need for a whole variety of assisting substances to enable anal sex to be enjoyable is the norm not the exception.
I stopped doing it because I realised that if you can't enjoy or even physically manage an activity without a whole heap of assistive and dissociative agents, then it's probably not an activity that is in itself fun, pleasurable or safe.