Aw, what a cutie! I'm not sure if he's overweight or not, but my parents have a yorkie that has a more "stout" build. And she just always appears to be chunky.
And don't feel like a bad mommy! It's obvious you care for him so much...accidents happen. When Miles was little, I took him to Target with me. I had my hands full in the parking lot, and set him on a lawn chair I had just bought...I just needed to get some stuff into the car. I took my eye off him for LITERALLY one second, and he freaked out and fell off...onto the hard surface of a parking lot. I felt so so so so bad. He was fine, thank goodness, but I made about a million calls to the vet that day.
Just keep doing what you're doing. The first couple weeks are a LOT of work with the potty training, sleep deprivation, etc. But the more work you put in now, the sooner it will get easier. I think in a couple weeks you'll find that you feel normal again.