I hope my coming will not put you to any inconvenience
I offer my most sincere apology
I beg you will excuse me
I regret it more than I can express
Excuse me for having detained you so long
I warmly regret being unable to oblige you
I must apologize for my stupidity
This is quite unforgivable, I fear
I am very awkward, pray forgive me
I regret that I cannot assist you