[yorkiemini]

I have had experience with your situation.
A behavioralist is probably a good idea IF they are well trained and know terriers and small dogs.

No one has discussed medication.
Medication and basic obedience training worked very well for me. It took may hours of work, but sounds like you are already investing lots of time and effort with your pup. I applaud you for that ! BIG TIME!

I would ask that be open to using medication along with your training. They key however is finding knowledgable individuals to help and I am aware that many out themselves forth as experts that are really not. So be prepared to work through that. I was so desperate for help that I got a lot of bad advice and and experts who were not expertness.

All of that investment finally came together to give me a sweet loving Yorkie once the anxiety issues were positively addressed. I will say again, medication was key for a while.