5 year old german shepherd

Question
I have a gsd who until recently has been a very obedient and loving animal.  About two months ago she attacked our black lab for no apparent reason (other than possibly due to jealousy).  I tried to physically break up the fight, which was very hard to do.  Our lab had to have stitches on her front leg and I had bruises as a result.  Things seemed to be going smoothly until it happened again yesterday.  The situation was the same.  I had been getting out of bed when the attack occurred.  She aggressively fought with my lab again.  Again I tried to break it up.  It is now to the point where I cannot trust her at all and I think we are going to have to get rid of her.  This has caused great anxiety for me as I have never been in this type of a situation before.  She was our third gsd and I am very attached to her but her behavior is unacceptable.  What should a person  do with this situation.  She is unadoptalble due to her aggressive behavior.   She is very attached to me and I love her but I cannot keep a dog with this kind of problem.

When we first bought her we took her for training and the trainer knew immediately that she had a mean streak within her.  This was apparent to him when she was 16 weeks old.

Any kind of help you could give me would be greatly appreciated.  

Answer
There are some cases where even the most experienced dog owners can't keep peace between to dogs, most often 2 females.  If the Lab is 3 or less, she may be maturing and challenging the the Shepherd for leadership.  Spaying helps and also obedience training.

If you conclude you must give her up, look at a Shepherd rescue.  You may find a rescue near you starting at http://www.akc.org/breeds/rescue.cfm  They may be able to retrain her some and place her in a single dog home.