7mo GSD female need some advice please

My 7mo GSD has been reactive since she started being able to walk outside (before I was carrying her everywhere) and it's been a few months of trying to consistently help her change her conditioned emotional reaction toward people and animals. It seemed like she was starting to do better, but recently a few people have approached us on our walks and since then, she has regressed so far back.

We're starting back at square one, but this time it's like no matter how far away the trigger is she instantly goes over her emotional threshold and she is on high alert and more reactive the rest of the walk.

Walking outside/running around outside is her primary source of exercise. I live in an apartment with no patio or backyard.

I feel defeated and like I've completely failed her. I don't know what else to do, I don't have the money for a professional trainer to work with us. I did shell out some cash for some online classes but this time around it's just not helping.

She's otherwise an amazing dog: so incredibly smart, playful, loving, goofy, and hilariously judgemental.

I can't imagine parting with her, but I don't want to be selfish and keep a dog that ends up just miserable because I can't help her. But because she's so young, I feel like she still has a high chance at an amazing life. I'm absolutely torn, emotionally exhausted, and feeling hopeless.

Does anyone have any advice for us?