Hi,
I am new to this forum as only have time recently to do anything online from my busy schedules.
Well, aggressive dogs just need to be taught to behave. Please, do not give up on your dog. The fact is, it's probably wondering what to do with itself too. Giving up would be the saddest choice.
You could go the behaviourist route, but have you tried looking into some good home training courses for dogs? Sometimes, nothing beats regularly reinforcing behaviours right at home. A behaviourist can advise you what to do, but behaviours need to be inculcated which may take some time. Unless you planning to board the dog, it may really stretch your budget.
Try browsing the bookstores for a good home training programme for dogs.
A trick i always use is to show my dog that I am 'friendly' to the person it was just try to nip at. Dogs take on the likes and dislikes of owners too.
Try to see how you can get educated on home training yourself before you give up. It may save you alot of time and money, and save your dog too.
Have a nice day.