I get that you REALLY want physical shapeshifting to exist. Yes, it?d be really cool to turn into a werebeast from the movies. But wanting it really badly isn?t going to change the laws of physics and other realities of this world.
Firstly comes where lore on shapeshifting likely comes from. This namely is trance, shamanic and animistic practices, soul-flight or shamanic journeying, etc. In concept this is things like becoming an animal in trance with a mask on. A great example is the the wolf cult of Livonia.
Lore (namely European witchcraft in the early modern period) and currently existing indigenous practices elsewhere are rife with these practices. Stories stem from this, rather than actual shape-shifting, and the very nature of how expirences become stories and myths is exaggeration.
An ancient indigenous story about how an ancestor turned into a fox does not prove that people today can shapeshift into foxes. Neither would a thousand of these stories. They can be beautiful pieces of cultural creativity and carry a great deal of meaning, and may point to spiritual practices, but they are not evidence of the physical reality of shapeshifting.
Secondly, physical shape-shifting is theoretically impossible.
Problems like conversion of mass (a 150 pound person cannot become anything greater or lower than that without consuming biomass. This could not happen instantaneously), where energy for this come from (the sheer amount of calories needed for one transformation would well surpass the calories consumed within days, let alone turning back. Even if there was these calories, the body temperature would rise with burned calories well above livable levels.), the issue of DNA and the fact of specialized cells in every organism.
Beyond that we have more bacteria in our bodies than actual human tissue, and these are specialized for humans. If you took human gut flora, and replaced it with a wolfs gut flora, in a human body, that human would die.
Also, even if all these issues could be solved or bypassed, the human body would go into shock at the shifting of a body and no doubt the pain would be immense enough to kill the said shifter.
Now, I can tell you right now I've had altered states of mind and out of body experiences as animals (in fact I recently went into to trance and had a spiritual experience as a snake). Again, many shamanic and animistic practices do this. These practices are not physical transformation. Explore those. That might be of help.