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So, this was delayed due to life reasons. Time to get back to it.
On my first library trip to find more details, and hopefully actual changeling myths instead of the summaries I had found online, I pretty much just got tales from England and Ireland. It's a public library not a university one so I guess I had unrealistic expectations as to the variety of its contents. But I did find some of what I was actually looking for: transcriptions of actual myths.
The best I found was a scholarly source with both written and oral traditions. Its contents were discouraging. None of the descriptions had the changelings possessing non-human forms, so nothing to match my shifts. Also none of them grew beyond early childhood, in the one story that had a family keeping a changeling for 20 years it was an (enormous) infant the whole time.
My other point of information for myself, the feeling that I've been placed in this body for a particular (unpleasant) purpose, matches nothing in these stories. Where a reason for changelings was speculated on it was fairly benign. Fairies being unable to have children, care from humans for the elderly, fairy love of beautiful children, and a specific desire for human breast milk all came up, but none of those click with the thoughts that bubble up that I consider my vague "instructions". In fact several of them involve older fae coming to the human world to replace children, which would include memories of their lives beforehand. I have no such kin memories and don't expect to develop any.
When it comes to physical descriptions, the makers of folklore and I are really working at cross purposes. They describe changelings as being ugly, ill behaved, learning to walk and speak late or not at all, and dying young. I can dismiss these because people are trying to explain the world around them, understand why the had a child with a disability or illness, not give explanatory descriptions to someone trying to figure out why they don't feel human. I don't feel like I can use these "mundane given supernatural explanations" stories for understanding my own changeling identity.
Which means the stories I've found up to this point only give me a strong pinging of similarity, and not much else (plus the intellectual curiosity but yeah)
I realized this is probably a better place than the forums for musing on and documenting my research into kin stuff. So that's what I'm going to do. This will take a while because in addition to what I've already posted I have another chunk from this past week and further thoughts on what I had before. So yeah.
Starting off in my research, I wanted to have a solid foundation for what I'm looking for, to minimise being influenced by whatever I saw (sort of how med students start diagnosing themselves with all the things). My phantom shifts are important... but creatures with wings and horns are wayyyy too common for that to be a helpful starting point. And going with the specifics, the wings are skeletal and batlike, got me a bunch of nothing.
I really feel the need to give the changeling aspect of my identity primacy. Whatever it is I turn out to be kin to, those feelings came first. I am not the original or intended owner of this body. I have a purpose in being placed here. These are the thoughts that come out of my subconscious in a voice not my own. Feelings I've had since early adolescence. I identified as a changeling for years before otherkin. Shifting came later.
So what I've been doing is looking into baby-swapping myths and trying to match that up with my shifts and other information. That's my starting point. End part the first, more later.