How can I get my two dogs to get along?

I have a yorkie and she s 7 years old. The last dog she lived with died 2 years ago and she s been without dog friends all that time. Even when we had the old dog, the yorkie (Lily) would sometimes snap at her if we gave her more attention, so Lily already had jealousy issues. Now, we recently brought a new dog into the home. He s almost two years old and probably two times as big as her. But Lily always tries to attack him! We ve unfortunately had to resort to a muzzle after failure on top of failure of introducing them normally, but even still she tries to snap at him and attack him. Her growling and aggressive behavior isn t improving, but we aren t going to give the new guy up either. What can we do?