Lets talk anal!!!!

Alright guys im no expert so i need advise from those who are pros at it lol....why does it hurt so bad!? Ok so we use plenty of lube problem is the penetration process i always end up backing off becuase its painful.till the other day.so i noticed when it was semi hard it wasnt as painful as it is when its fully hard ..what can i do so its not so painful!? any and all advise is helpful