What's her name ?
As Chris says , she is under socialised and fearful. Although she was making progress , each time a dog jumped in her face or sniffed too long , it caused her stress and stress builds up if this happens frequently hence her becoming more defensive. Behaviour is an outward expression of our emotions. By lunging and barking , she isn't showing confidence , she is fearful and is acting like this to keep the other dogs away. You are doing the right thing by keeping to the side lines and also you can block other dogs unless she knows them or turn away and go on another direction if they come to close or she starts to react. (Some dogs are very rude and pushy, its really annoying that owners let their dogs run willy nily around the park being a nuisance.)
You can also use food treats if she will take them or a toy to reward her if she is calm. This also works by changing how she feels towards other dogs. If she had something really yummy when she sees another dog , she will associate them some nice with them rather than something to be feared.
This type of agression is common so dont despair.
Look up on line about
Desensitising and Counterconditioning
BAT - behavioural adjustment training . (greisha Sterwart)
LAT - look at that .