Have you considered trying a Thundershirt? You can get them from Amazon or Thundershirt.com. I'm sure some pet stores sell them too. I have a 4 year old Yorkie/Poodle mix that I've had for a year. I'm pretty sure she had been abused by her previous owners or she just was never properly socialized. She's a very anxious dog and she's terrified of men. She's had a Thundershirt for about 6 months that she wears pretty much all of the time. I won't say it's helped her 100% but it definitely seems to have lessened her anxiety. This is Lily in her Thundershirt.
As far as getting another dog, have you thought about getting a male Yorkie instead of a female. I have 2 boy Yorkies and 2 girl Yorkies and maybe it's just my group but both of my girls get along better with the boys than they do each other.