I have a very limited knowledge of Sullivans methods, though am fairly knowledgable about the ideology behind it.
So you're attempting to teach your little Boston to "heel," correct?
If that is the case, start inside. When walking around with the dog, any time the dog forges forward bring the dog back to your side with a leash correction, while continuing to walk with the dog. Once he has stopped forging past your left leg start the excercise outside.
I cannot stress that you should be in control when you leave the house. Wether that be a SitStay, DownStay or a calm dog. I've got 2 Bostons myself who are extremely energetic motivated animals. For control to exist you need to solidify and excercise the control before you move on with any other excercises.
It's hard to tell without seeing the little bugger, where exactly you're going wrong with the training. Sorry, wish I could help more