My shepherd mix completely loses it when she sees other dogs on leash - where do I start?

My 2-year-old shepherd mix Rosie is great at home and at the dog park, but the second she sees another dog while she's on leash she goes completely ballistic. Barking, lunging, straining at the end of the leash - it's embarrassing and honestly a little scary because she's 55 pounds and strong. People on the sidewalk definitely judge me, which makes it worse.

The weird part is she's actually fine once she's off leash and can greet dogs normally. So i know she's not actually aggressive, it's something about the leash situation specifically. She gets so frustrated or scared or something that she can't hold it together. I've tried just walking past dogs and giving treats but by that point she's already in full meltdown mode and completely ignoring the treats.

Anyone dealt with this and actually made progress? Looking for where to start - training classes, specific methods, anything. I feel like i'm doing everything wrong.