I can sooo relate to you. I recently went through this same delima. I have a 2 year old Yorkie and we 'rescued' a Maltese/Shih tzu miz that her owners had given up on and didn't want. Maddie had been our only baby and she knew she was boss.
I had no idea what to do and just brought the newbie home. I did introduce them outside in the yard. Maddie just kept her distance and pouted. It was so hard for me
Maddie even didn't want to have anything to do with me for a few days while I was involved with the new pup.
I had several people on here tell me that they would eventually get along. Well, three weeks later, we are making excellent progress. I made sure Maddie got lots of attention and extral love. Made sure to take her along on errands while leaving newbie at home. She eventually stopped growling when Cooper came into sight and they actually play some. Cooper gets on Maddie's nerves a little so I try to always be involved with them while they play.
I would sit in the floor and hold Cooper and let Maddie sniff her. Puppies tend to be a little 'aggravating' to adult dogs. By me holding her, Maddie was able to get close to her without her jumping all over her face.
I also, always fed Maddie first and didnt let the new puppy get into her bowl, this happened once and Maddie lost it!
Cooper is always fed in her crate after Maddie's food has been placed for her.
After a good play session, I would give the girls a treat, Maddie first of course. They are doing much better. Hubby and I are just trying to adjust to 'puppy days' again.
Good luck! Let me know how it goes and if you have any questions!