How Can I Correct My Dog's Social Anxiety?

I have an 11 year old border-collie/ lab mix. As of a few years ago, I had realized my mistakes in training her as she would bark and be very anxious in social situations, even with meeting new people. (Thank you, Caesar Milan) But I am still working on being a good pack leader.

I've made a lot of progress, and she can now comfortably meet new people (after a short barking introduction, which I decided not to train out of her because I'm a young woman living alone and would like to deter ill-meaning guests) and she no longer barks at people who pass us on the sidewalk.

For the most part, she can coexist with other dogs, which is a huge improvement. Upon introduction, though, she still barks and is very uncomfortable and avoids getting her privates sniffed. After a few minutes, she is fine and calms down and just lays around.

BUT with higher energy, more aggressive dogs, she's prone to getting bit because she is still so uncomfortable.

I took her with me to house sit two very small dogs. One was very old and very low energy. The other was a young high-energy thing that very obviously had claimed the house. Neither were very well trained and did their business all around the house and the younger dog was constantly trying to dominate and jumped all over.

The intro, as I figured, was a little rocky but it quickly quieted down and I felt comfortable enough to leave them alone together while I got my things from the car, which went very well. Later that night, though, my dog was sniffing the smaller dog while it laid next to me on the couch, and the small dog bit her, which busted out my dog's anxiety again and she whined after the initial shock but I quickly diffused it. Except it happened a second time, and I think that it was due to my dog's anxious behavior.

What are some ways that I can become a more confident leader so that my dog is less anxious? I'm concerned that I'll lose all of my progress if she continues to be attacked because she's scared around other dogs. Help?