Here are some my grammar comments.

“It wasn’t a suggestion slut. Come here."
“It wasn’t a suggestion, slut. Come here." If you don't use the comma, it sounds like there is something called a "suggestion slut," that is, a slut for suggestions who gets aroused whenever someone makes a suggestion.

My insides rolled and my mind raced, quickly trying to amend my mistake.

To amend means to add something to something else that was completed. But I think you mean "fix," i.e., "mend."

“Head to the floor slut” she said.
"Head to the floor, slut." Do you see that it's your name? But without the comma it could mean your head is being ordered to the slut. What kind of slut? A floor slut.

Not knowing what to do, I waited, my body vibrating with anticipation until “Well?”
You need the ellipsis "..." This is used when words have been removed. Hypothetically, you removed the words "my cunt exploded" or something to that effect. But no, you removed those words, conveying the sense of having your own thoughts interrupted. So I'd write it like this:

Not knowing what to do, I waited, my body vibrating with anticipation until... “Well?”

I gasped, trying to suppress an incorrigible whimper.
Incorrigible means unable to be corrected. For example, an incorrigible whore is one who can't stop whoring, implying of course that whores are bad, even though they are awesome. But how could your whimper be corrected? In fact, you were trying and may even have succeeded in suppressing the whimper, so there wouldn't be anything to correct.

I wondered what I was to do; do I beg, do I…?
I would add the question mark.

Her voice cut my thoughts off, “You will clean my boot when you’re finished.”
This is the place for a period rather than a comma. You would use the comma if the first half was "she said." But, I say a period is sufficient, even though you are making a quote. If it is clear who is saying something, a sentence in quotes with no "she said" is still good. Here, the context tells the reader exactly who said it.

She didn’t say anything as I struggled to stay put, mostly.
The "mostly" dangles. I can't be sure if she didn't say anything mostly or if you struggled mostly. The main subject is she, so I would think "mostly" applied to the main subject. But "mostly" is all the way at the end of the sentence. I'm so confused!

I gasped a “thank you” and didn’t wait a moment before resuming my fervent humping.
Here, you used a quote with no comma and I agree. Short phrases like this, especially phrases introduced by "a" are almost like single noun words.

Within moment’s I was close again, my breath raspy and my movements slightly jerky.
No apostrophe on "moments." It's not possessive.

I disagree about the comment above on semicolons. I like them. Exclamation marks, however...