If the stop and turn isn't working, you need another method.
My class get taught to walk using two hands on a lead method. I will see if I can explain it well enough :)
You need your dog on your left side. Have your right hand (with the lead looped round your wrist) on your belly, and your left hand on your left leg with the lead under it : like this but you want your left hand lower down so it holds the lead closer to you
(this is harder to explain, but you almost want to weave the lead in your fingers, so if you lift your hand, you shorter the lead automatically, but its not essential!)
In the left hand, you can hide a titbit under it to keep your dog in the right place, which you can eventually drop, or keep on a walk for a surprise :)
With that position, your dog has as much or little lead as you give her. You set off with the lead fairly loose next to you, and tighten it up as you need to - using the command you want - for my dogs its "With Me" - as you tighten it up, the dog comes into the right place, and you can reward her :) If your dog still insists on surging forward, you can spin in place ("close close close close" in my house) then reward. Because you have the lead in two places, there is less slack in the lead, so the dog doesn't have lead to pull forward on to. You can tighten it up by raising the hand on your belly to your bra (!) and/or bringing the hand on your leg up, without yanking or wrenching.
To start with, you'll feel all fingers/thumbs and lead, but if you can picture what I mean, it soon makes sense.
Its easiest if you first do this as a completely separate exercise to a walk, and if you walk in a large clockwise circle. Once the dog is walking nicely like this, you can then start to go anti-clockwise (this'll teach her to watch where you're walking, as anti-clockwise will have you walking into her a little bit) and then I'd work on going straight.
The good thing with this is you can slowly make the lead longer over time so the dog can walk further in front of you without pulling, so you aren't restricted to direct beside you forever.
This is just one method though, and it may be easier to find another trainer locally :)