Ah lauren! This is just so terrible! Big hugs to you, girl!
Unfortunately, difficult managers are like mushrooms - they're everywhere! We kinda just have to figure out how to survive them. Is there a way some of the other groomers in the salon can vouch for you? Like when you had kidney infection and all that. Maybe Janet-witch will listen to the other groomers. Or about training you... maybe they can put her on the spot by asking her why you're not earning commission yet.
Also, you can ask her straight out, "Hey Janet, may I bring Delilah after my shift tomorrow to groom? The last time I took her in was X days ago...". Then, it will look really bad on her if she says No...
Also, is there another big box pet store there, like a PetSmart? Now that you got some experience down your belt, you can use that in your application. Or, if there's another PetCo close to you, you can probably get a transfer.
In any case, just keep trucking. I truly believe in Karma - what goes around, comes around. So, don't let Janet-witch bother you. You are a good person and I like that you recognize that about you. You know she's just one of those idiot managers we have put up with. Grrrr.