This 17th century English idiom could fit, Wiktionary says:
neither fish nor fowl
(idiomatic) Something or someone which is not easily categorized; something or someone that does not rightly belong or fit well in a given group or situation
Alternatively, Oxford Dictionaries suggest
neither one thing nor the other
Not clearly either of two things: Sam stands on the cusp, neither one thing nor the other
I do like the term cusp which I think fits in with the OP's description. He or she may be on the cusp of adulthood, emotionally and/or physically.
Conflicted, describes the emotion.
confused or worried because you cannot choose between very different ideas, feelings, or beliefs, and do not know what to do or believe