I'm an only child too and had a very similar childhood. I'm now in my late 40s and only just started therapy, which I've found extremely helpful. I don't have a clinical term or diagnosis, but have covered aspects of this under words like regression, life span integration - try these in Google Scholar. I have to say that my approach is to embrace it rather than force myself to be an adult in ways I don't want. I work full time, drive, own my house, cook and clean and so on, but I don't want to date or do certain adult type things. Acknowledging and acting on that has been liberating. I don't mean I wear little dresses and play with dolls, but I do colour, read and watch Disney and things I miss. I don't care - it's fun!