I couldn't do it for the longest time either.  i thought i never would.  then i was with a guy that actually cared to find what i like.  it was like the mental block was lifted!  i get off more often than not now.  you just have to know what you like and go for it.  just get lost in the feelings.  deep grinding works well for me, but the g spot is the best. a little messy at times, but definitely worth it.