Things Aren't Going Well With My Dog

I got a dog a few months ago thinking he'd make a good companion. Despite the person assuring me the dog wouldn't get very large and would have a sweet disposition he's turning out to be the exact opposite. He's pushing 20lbs. and goes from zero to 150 in the blink of an eye. Last night, because he didn't want to go into his crate for crapping on the rug he attacked me. My hands are severely bitten and he drew blood in several places. Despite all this I really love this dog. 95% of the time he's sweet and loving. Its the rest of the time that's a problem. I've gotten some great advice from face book friends but I'm afraid this is his disposition and there's not much that can be done. I'm waiting to talk to a dog trainer to see if she's got some advice for me. It would break my heart to give the dog up. By the way, he's a Lhasa Apso - a lion dog. Now I understand how such little dogs could serve as palace guards in Tibet!