Getting a stubborn ex driving pony to lunge?

[Montyforever]

Currently helping to get a friends very overweight welshie fit, she's only ridden once a week. She's a very sweet and intelligent little pony but i cant seem to switch her out of "driving mode" she's absolutely fantastic to long rein out on the roads/paths she's forward going and behaves perfectly
Tried lunging her the traditional way first but she just turns in and walks straight back to me, don't think she understands what im asking her to do! So have been using two lines which means i can pull her back out if she starts coming in but we are still lacking oomph! She's realising that i want her out on a circle which is already a step in the right direction but just isn't forward at all! I can get half a circle of trot now but cant keep her going as obviously if she's been driven a line flapping round her bum isn't going to push her forward. Im going to try holding a whip as well but have a feeling ill get in a muddle and given how stubborn she is don't know if it will help much anything else i could try?
She's a sweet little pony, still young so got plenty of time to learn and there's no rush now i can long rein her out her for a bit of exercise but lunging is something her owner could do more confidently than longreining and would be a harder workout so would help with shifting the pounds

ETA - going to keep an eye out for a trap/harness but need to work with what we've got at the mo!

 

[katastrophykat]

Use a roller with rings of you can't get a harness, and pop the reins through the highest ones. Long rein in an open space, her her going on, start some shallow loops to gain understanding then make them longer/less shallow until you have a circle. The reins through the top is more the 'normal' place for a driver, and means that they don't have to go around the outside. It also reduces movement through the reins as I always wonder if when lunging with a rein behind, the movement of the back legs must have an effect on the rein command. If you have a sensitive mouthed pony/horse it's bound to confuse them!

 

[ChesnutsRoasting]

I'd use a helper (on the outside) to guide her. She just sounds a bit confused!