As for your copyright, the terms of service on websites deal with this, making certain that the content you provide is licensed to them (it has to be, or they wouldn't even be able to distribute it at all), and can spell out altered terms to avoid the problem of someone insisting their comments be deleted later. Further, even without extra clauses, I would argue for copyright purposes the "work" is the conversation, not your words, and so any lack of clear ownership should either argue for the entire conversation to be deleted due to one objector (which I think might be more likely by law, and what the terms of service are dealing with) or none of it to be able to be deleted until everyone agrees (which seems more clearly correct and what the terms should state if deletion is to be allowed); I can't come up with a scenario where it seems OK for you to delete just your part unilaterally.
You decided to go to a public forum and contribute to a public conversation. The fact that you said something meant that some people replied to you and others upvoted you instead of saying the same thing, making your comments load bearing and structural. You going back later and deleting what you said changes the context of what I said. For a great (related, though slightly different) example, seek out the account on reddit whose name was something like "edited_to_make_you_look_dumb", which would later edit what they said to make all people who responded to it look really dumb. The two hour window on Hacker News feels about perfect to me, giving time to fix typos, make minor structure and formatting fixes, and even delete things while other people still have time to react and adjust, but then locking the content in for the later record when the content becomes part of the archives.
When you go on a redact and purge, you are destroying something that not just that you created, but something that tons of other people took part in, and you are individually revising the historical record. One may as well argue "I also want everything I said in public comment at last week's City Council meeting redacted from the minutes and I want everything I said at last week's trial redacted from the transcript". This is how you should treat saying something in public as part of a public dialog: you said something in public, and now are part of the public record. Again: if you weren't prepared to say something in public, don't say it in public.
And honestly, I would much rather you not contribute to a conversation if you are going to come around later asking for your part to be deleted. It is as if we are going to paint a mural together, but for some reason every five years you go around finding all the murals you helped with and take back all your paint. If you hadn't have been there at all, someone else might have helped more, and it isn't at all clear our mural would be drastically worse off: but you removing your paint destroys the mural and invalidates all of the work of everyone else involved. It wasn't "your paint", it was "our mural", and you shouldn't get to unilaterally alter it :/.