Hi there,
congratulation
s on your new addition, she is beautiful.
Your problem seems to be really minor, since you say she already stays close when off lead.
One thing you can do is just simply stop as soon as the leash gets tight and once she backs up and the leash is loose again you continue to walk. At the beginning you'll get more stops than steps but she'll get it.
Another thing you can do is walk backwards as soon as the leash gets tight and continue forward as soon as the leash is loose.
You don't even have to treat her because the reward will be to continue the walk.
Good luck, you'll get there.