Hi
What you have to bare in mind here is that terriers have or should have built in determination to never back down because of the job they were bred to do.
There is one pet forum that is very popular, uk based and has members who are professional behaviourists, vets, vetenary nurses, dog charity staff and many 1000's of ordinary members, most of which are very seasoned dog owners.
From there you can get a lot of direct sound advice or help with finding a real behaviourist in your area. Just be aware (law may have changed now) but anyone can set up a website and call themselves a behaviourist or trainer.
One thing I see a lot of people say with these situations is the first thing to do is have the dog checked over by a vet to rule out any medical condition that is making the dog "angry"
Also, dog on dog aggression in no way means the dog will want to do the same to humans.