Jackie is absolutely right! My youngster, Ra Kismet, was attacked by another dog jumping the fence. Up to that point he was fine with other dogs, then he developed a fear based over reaction big time! I worked for a week in my own garden, just click treating him for walking quietly along side me. Then we went outside and only went a few feet either side of my own gate. No dogs around at all, as he was scared of just going outside. Step by step we went a little further, and I click/treated him all the time. (He is clicker trained so understood the clicker). When we did hear other dogs, further down the road, I asked him to 'look at me', something we had practiced in my own garden for a week and click/treat. When he was really good, like not over reacting to a dog barking he got the JACKPOT, which means he was allowed to eat the entire contents of the main pocket of the trick bag (I had some treats in another pocket of the bag of course). Gradually, over several months, he became less and less afraid and now we're almost able to pass dogs coming head on without any reaction. It takes time, and as Jackie has already said so eloquently, it's fear based and the last thing you want to do is make Prissy even more afraid. Had I got angry with my boy, or hit him, I hate to think what he'd be like now. Instead he's now a joy to walk once more.
Keep working slowly, step by step and do check out all the videos, threads etc. Really study them and learn, don't lose heart or patience, Prissy is a little dog who needs to know you 'have her back' when those big scary dogs are around, not that she's going to be punished further.
You may find a halti/gentle leader will work with her. Not all dogs like it, mine does, but with the band around the nose it gives them a feeling of security, like you're helping them out, rather than they having to be in charge of things. This is NOT alpha leader of the pack at all, but rather reassurance that you've 'got their back' and they can relax knowing you're protecting them. If you look at the Thunder Wrap site, you'll see they have a special nose band, and it explains how and why this works. It's basically like a halti/gentle leader.