Mike's giving some really great advice. He's right, they need to decide which of them is in charge (after you) and it needs to work out on their terms. Whoever ends up being submissive will actually be happier once it's worked out. Please, don't feel bad! Dogs are pack animals, and once they work it out you will see some amazing things.
I have a 15 year old dog. Two years ago I brought Ginger in as a pup and he wasn't happy at all. But, they worked it out, and the old guy even tries to play sometimes. They never fight over food, bones, etc (they used to in the beginning)...and seem to have a good relationship.
When I brought Bernie home a few weeks ago, we thought Kirby was going to eat him. But, beyond breaking up the first fight (which was getting bad quickly) we stayed out of it. By the third day they were sharing a blanket.
And Ginger has fallen madly in love...
(but even she and Bernie had a couple of battles at first)
They can and will work it out. Hang in there.
Oh, and did I mention, she's adorable!!!