Welcome to Chaz...
You think she is German Shepherd and Collie - when I read your dogs behaviors I immediately thought "sounds like a herding dog" lol
I know a couple Collies (down the road from me, actually) that used to have the same exact behavior (sounds like it anyway). The nipping at the feet especially...
What kind of commands / obedience have you done with your dog??
My first suggestion would be to enroll her in a training class if she's never been. Not only can a trainer work with you and her, but its good for socialization as well
I would stop in your tracks and ignore her completely until she stops, and then praise her when she is not nipping at you. Sometimes we don't realize that we're encouraging the behavior by jerking away, flailing our arms and feet around, etc. Shouting and holding her down will NOT work - which I assume you already can tell. It has the potential to make her behavior worse, because you are getting physical with her, and she will probably get frustrated by you holding her down.
If she is leaving marks on you, then obviously I would try something else. Does she know "leave it" or a similar command? I would highly recommend teaching and reinforcing this command
Article w/video as it will be useful on your walk
Meanwhile, when she starts nipping at you I would go "eh-eh!" sternly, to interrupt her behavior. Then I would ask for a more appropriate behavior, for example, asking for a sit. Or 'watch me' (I have yet to meet a dog who can watch your face and bite at your heels at the same time lol), a "heel" with a couple turns to keep her attention and keep her busy
With a mix of both of those breeds I think it is safe to assume that your dog has a lot of energy - make sure she is getting enough exercise. "
A tired dog is a good dog" A bored dog, or one with pent-up energy is one who is going to find something to do, often something we consider inappropriate, or destructive.
I hope that helps a little. I know more Chazzers will join in with some good advice. Again, Welcome to Chazhound - Do you have any pics of your girl? What's her name, btw?