My 8 Month Old Dog Challenges Me. What Should I Do?

Question from Erica: My 8 month old male Standard Poodle has recently challenged me by biting me twice. We adopted him from a family that neglected him during the first 16 weeks of his life, leaving him unsocialized and with some behavioral issues. When we brought him home at 4 months, he had separation anxiety and if we left him alone he went crazy... barking excessively non-stop and he still does. We try and take him on walks and to dog parks as much as possible. He gets along with other dogs great but he is still very shy towards strangers. He will not allow anyone to pet him, except my husband and my kids. He uses a prong collar and has had no problem with it, until recently. I had a couple of guests over one day. He got too excited, so I grabbed him by the prong collar with my hand and he turned his neck around and tried to bite me while making a huge fuss, whining and barking. So, I let go of it thinking that was all that was needed. Today, as he was outside, he saw a cat. I immediately grabbed him, again by the prong, this time he attacked me. What should I do?