Suggestions?

Train her. She is only a young puppy. You said you knew when you got her that Boxers are hyper dogs. You know why the dogs you've been around were managable? Because their owners trained them.

She needs to be an inside dog to learn the pack rules. Outside dogs have a difficult time understanding that. Keeping her outside, and reducing her size of yard, is just going to make it worse. She needs to run, she needs to play with other dogs, and most of all she needs to be with her pack - your family, to learn the important behavior aspects in life.

Keeping her outside is your #1 problem.
Bring her inside, crate train her (for when you are gone, ONLY,) and allow her to learn the rules of a pack.
I can assure you that there are many of us here with experience with puppies that can help you. Heck, we can even learn together because I've got a 6 month old here myself.

Selling her just sounds ridiculous. Who cares about money when it comes to this?