I have a couple of thoughts on this subject. If you are 17 you are a legal minor and you cannot own property. Dogs are property. It's your parents' dog. They can't afford a better trainer?
My business partner and my mother both owned Springers. I have owned at least 2 dogs all of my adult life. Both Springers were nasty, possessive of their belongings, a trial to be around. My mothers' dog bit my mother! My partners' dog liked one of his children and hated the other 3. His dog actually got in my car one day, wouldn't get out, I couldn't get him out, I had to drive to her school and pick her up, drive her home, she got the dog out of my car and into the house. I am not fond of the breed.
Your dog sounds like it needs consistent training. I have found that consistency is the key. What is not good one day can't be good the next. Do the dog's "rules" fluctuate?
Has he been thoroughly checked by a Vet? My dog was crotchety (for lack of a better word), and she's 130 pounds. I couldn't live with a crotchety dog. I took her to the Vet and he discovered a very large cyst on the back of her neck which was rubbed by the harness when I walked her. Had the surgery, removed the cyst, she went back to herself. I brush my dogs, but I never saw the cyst.
I rescued a dog from a fighting ring (she was a teaser, not a fighter), pulled on lead and I bought a fitted harness. She COULDN'T pull, and I had much better control of her. Once she was trained, she went back to a regular harness. My dogs are both large, and I would never walk them on leashes.
I appreciate your sadness. This has got to be tough. The truth is if you can't trust the dog, if the dog is wearing out its welcome, perhaps it needs to be rehomed. It could be a question of bad breeding or just about anything else. I volunteer in rescue, and I don't judge people. Sometimes a certain dog does NOT work out.
Did your family research the breed before buying the dog? I see people almost every day who buy a breed without research and discover it's too active or too territorial or too headstrong.