I posted with the same problem with our Dillon a short while ago. Absolutely desperate and with all the same 'symptoms' as Millie! Dillon is also 6 months old now.
We tried all the suggestions given, stopping turning around, waiting patiently, throwing treats.... all to no avail. What made it particularly annoying is that on an off-lead walk he never 'stopped' he would go on and on if allowed to...no problem! He is not a nervous dog, loves other dogs/people, although he does appear cautious at times.
However, recently, Dillon has had a throat infection and we were advised to avoid a collar while he was on medication so there would be no strain (fat chance!) on his neck when walking (lol!) or becoming too excited (as this made him cough). So.....we invested in a harness, something we had been considering anyway due to the lack of walking, in the hope it might improve things. Guess what...it has! For the past four days we have been out once or twice a day for at least 30 mins each time with no stopping or at least that is the case if we are together......if I am on my own he has put the brakes on a couple of times but it seems a little easier to jolly him along. We are hoping that this continues and eventually he will come to enjoy his walks as much as his off-lead adventures.
I must say this behaviour has really taken us by surprise I have never heard of a dog not loving their walks before - it was certainly not the case with our 3 previous doggies! We too love to walk and it was really the main reason we decided the time was right for another dog so this has been a real concern for us

Anyway maybe try the harness?