teenager gets overstimulated during heel training sessions

I have a (freshly) 1yr old German Shepherd.

Mostly, her adolescence phase has been fine so far. She’s a bit more independent, and I’ve noticed her drive has increased, but she still engages with me and listens when necessary.

The issue is when it’s actually time for training sessions. We’ve done these sessions at least every other day since we got her at 12wks old. Ten to fifteen minutes, twice a day. Usually I’m building off of basic obedience— sit, down, reinforcing her implied stay, recall.

Recently, however… 🤦‍♂️ Right now, I’ve basically only focused on her heel and the occasional recall. I try not to overwhelm her with commands. Heel is something I’ve been attempting to teach since we got her, but it’s failed to click despite talking to trainers and using different methods.

We will get about five minutes into a session of heel, with the occasional sit or down thrown in to keep her confident about the commands she does know, and then— she gets incredibly overstimulated or frustrated with the training. I’m talking barking at me, nipping at my fingers to try and get the treats without following commands, and even throwing herself down on her side and shoving my ankles between her teeth (which is redirected into biting a toy. She is a mouthy breed, I understand this is how German Shepherds typically redirect strong emotions, I just wish it wasn’t my body).

I’m not really sure what to do, though. She needs to learn heel. I’m not sure what I’m doing wrong or if I’m doing anything wrong in the first place, but the last thing I want is for her to lose confidence and not find our training sessions as enjoyable and engaging as they used to be. I’ve held off training it for this exact reason, but now that she’s older and we hike and go out more, it’s something she needs to learn.

Any advice or suggestions? Anything is appreciated at this point as my ankles and I are getting quite desperate. Thank you!

EDIT: How I am currently training heel is having her on my left, her other side against a flat wall so she stays parallel with her shoulder aligned with my leg. I lure her two, three steps forward, and then mark and reward.