My Bull Mastiff x Sharpei puppy won't listen to me when i try to dicipline him. What am I doing wrong????????

I have a 7 month old bull mastiff x sharpei puppy who just won't listen to me when I try to dicipline him!!! He responds to sit, and sort of responds to come, but won't listen when I say stay, he keeps jumping up, barking at stupid things, and chewing every god damn thing in sight!!!!!!!! I live in a rental property now, and I'm almost at the end of my tether!!!! I can't stand this much longer. He listens to my fiancee, but not to me! WHAT AM I DOING WRONG!?!??!?!?!?!