Much to my regret, I am unable to…

让我感到极为遗憾的是自己不能……

Much to my regret, I didn't go with them.

非常遗憾，我没有同他们一起去。

Much to my regret, I can't accept your invitation.

很遗憾， 我不能接受你的邀请。

Much to my regret, I can't accept your invitation.

很抱歉，我不能接受你的邀请。

Much to my regret I am unable to accept your invitation.

很抱歉，我不能接受你的邀请。