This is more of a voice-acting question than a singing question, but I thought I'd ask you guys.

I am playing a character for a video who has this cartoonish, nagging, high-pitched voice, but I don't know how to do it without damaging my voice. I want it to have this nasal sound, but it hurts my vocal chords. I've practiced diaphragmatic breathing for years, but I can't do this kind of voice through my diaphragm. Please, help.
What you may be doing is tightening your throat to squeeze the voice out. What you really need to do is send it through the mask, as they say, the front of your face, that's what'll give it that nasal sound. There is a give and take, it'll sound quite different, however, with practice, you should be able to get closer to the voice. Work on relaxing your throat, first, as you should never squeeze, it should, hopefully, be easy once you've found the right placement for the voice.
nasal has to be pharyngeal. Keep your voice relaxed, sing with a big smile and small mouth opening, focus on sound at the back of your nose, and try to imagine a 'bunny voice', or tweety bird voice.

If you try to use a large wide voice position, you aren't gonna get that nasality.
Just gonna casually namedrop my podcast, Rotten Playground (it's in my sig). We tend to do this weird high voice all the time. I'm not quite sure how it's done, but it feels like it comes from further up the throat than normal talking. It's not really any extra exertion than talking normally, unless we do it for more than a minute at once. I think (though I don't know if this is right or not) what we're doing is pretty closely related to either head voice or falsetto, but I couldn't say for sure.

We do a beautiful rendition of Sweet Child O' Mine at about 1:00 in the video above - is that the kind of voice you were thinking of?
Rotten Playground
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as if you have anything better to do.

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