Hey there,
I'm sorry to hear things aren't going so well for you. I don't have any experience with Botox. I wish some of our other boardies who have would offer some advice...
It does sound like your sphincter may not be as relaxed as it should be since you are still having trouble with the re-tears, stinging and burning and it's been three weeks.
From what I've read on here from those that Botox worked for- it took about a week to a week and 1 and a 1/2 and they started having really good results.
If you haven't already read them, you may find Bambi's Botox threads helpful.
Please let us know how your follow up goes. I hope you are able to get some answers and relief from all this.

I wanted to give an update after 2 months.
So far so good, no more re-tears since the one I had 2 weeks after the surgery. I'm still taking miralax and dulcolax every day religiously. Some days it hurts a little when having a BM, but nothing like it used to.
I still have itching/burning on the outside and a little bit of the inside of the anus, but the doctor said it is most likely neuropathy and not the fissures. So I suppose Botox healed them =). I still have this new problem to deal with but overall I suppose the surgery was a success. I was hoping that it would clear this burning and itching that I'm feeling, but I'm certainly in a better place now than I was a few months ago.
I will update with my status at a later date.
Satisfaction level so far: 7/10 (itching and burning still a nuisance)
Pain: 2/10 (just the itching and burning)
BM: Soft and occasional minor pain
Optimism: 6/10