Dog Not Listening To Me

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Me and my dog, a 2 year old female German Sheppard are pretty cool. I usually pet her from time to time so she doesn't feel lonely. However, she has a bit of a problem where she loves eating her own crap. Usually I can get it without a problem. But, when she gets really happy, she wants to eat the poop bag and constantly jumps on me. I'm a bit short for a 15 year old, so she's hard to control. Is there a way where I can have her obey when I tell her to sit? Also, she has been trained and whenever my mom or older sister tells her to do something,she listens fine. It's just me. I don't want to make picking up dog poop feel like Fight Club.

(P.S, My sister's not living with us for a year, so I can't ask her for anything right now)