Muzzle's ok? Or last resort? Destructive dog

I have had a Standard Poodle for almost six years now, he is very well behaved intelligent all the good things anyway half a year back or so I heard of a dog who was stranded on the side of a hyway so I went and picked her up only to find she was a pit mix, now I don't necessarily buy into superstition but from the get go this dog was a menace, not bad per se just obstinant and opposed to being trained. But not terribly aggressive and super well behaved. When I was in the room. Now I dont live in an apartment but I still keep my dogs inside when I'm at work because I don't have a fence and I don't want them roaming the neighborhood. So I learned very quickly how destructive this dog was and it wasn't just when I was at work but as I slept she bit into the very mattress I was sleeping on, I have a four poster bed made of wood and it looks like a beaver got a hold of my bed frame.So I bought a muzzle for her and she stopped destroying everything as I slept so I kept it on her while I was at work and she was well behaved then as well, but still it's a muzzle and the back of the box says a dig should wear a muzzle at max 45 minutes and we're talking eight hour work days. So am I just the worst owner ever? Even if I were to try some training I've never heard of I wouldn't want to leave her alone without her muzzle for even thirty minutes cause that's all she would need to destroy an entire room. I'm at my wits end with this dog and it doesn't help that my poodle has never acted this way it just doesn't seem normal..