i’ve pulled my head back need advise

i was wondering if any of you boys could help me out i’m a bit confused. So i’ve successfully pulled my foreskin back while flaccid under warm water, my question is how far back is it supposed to go? because once i pull it back to fully reveal my head the skin kind of bundles up and is just a bit difficult to pull back up. and if somebody could answer my next question i would greatly appreciate it, why can i pull my skin back completely while flaccid under water but when i’m erect i still cannot pull it back over my head? i am currently using an 18mm ring but i would just want an explanation. thanks