Not 100% sure, but I think it's from one of the 
last two eps of the second OAV where "Tamahome" 
is trying to keep Miaka from returning to Taka, 
who is presently in the real world.  She's doing 
the whole, "Sorry, but I love him" thing.  

But I haven't seen the second OAVs in about a 
year, so I could be wrong.  Hope that will help 
your search.