I realized after uploading that my post was a duplicate, but I don't see a delete button. How do I delete my own post?

Use the inferior version/duplicate of ****** flag reason, and insert the superior version's ID where I put asterisks.

But if my post is flagged and removed, doesn't that add to my list of deleted posts, reducing my upload limit?

Also, separate but related issue: another post I uploaded recently was removed because someone else provided a better version less than a day afterwards. Why does this count against my upload limit as well? Why am I being punished for adding content to the site?

Yes deleted uploads reduce your upload limit. The purpose as I understand it is not so much to punish, but to motivate users to use due diligence in finding the best version of a post to upload. See howto:sites_and_sources_-_n_to_z#twitter.

aaronfranke said:
But if my post is flagged and removed, doesn't that add to my list of deleted posts, reducing my upload limit?

Also, separate but related issue: another post I uploaded recently was removed because someone else provided a better version less than a day afterwards. Why does this count against my upload limit as well? Why am I being punished for adding content to the site?

The treshold is extremely high.

You would need 40 deleted posts and zero approved posts for your upload limit to go 0. Also now consider that at first you have to wait after uploading of 10 posts and if all 10 get deleted and you still continue doing the exact same thing over and over again, you have to do that more than 4 times as the limit constantly decreases. Even after that you can still ask admin to look into it and raise it if the reason that has gone down is understantable.

Also approved posts keep the limit up, I have actually had users who were constantly deliberately generating more work for everyone on the site, but bit over half of their uploads were still acceptable, so their upload limit was actually manually reduced to 0 to prevent further bad uploads by them.

This is exactly the same issue we have with deleting artists own uploads for stuff like irrelevancy or quality: the immidiate feedback to your brain is negative even when the purpose is neutral. I can almost say with confidence that all accounts that have uploaded content here will most likely have at least one deletion which was out of accounts hands, were it inferior or takedown.

The upload limit is there to act as a buffer which is increased when it's known that user knows how to handle stuff. Prevents situations of new user immidiately uploading 100 pages doujin they scanned with only tagme and no pooling because "I'm new here please help uwu".