Is there an expression for the feeling of wishing you had met someone earlier?
Solution 1:
As far as I know, there isn't a word or expression in the English language that describes that specific phenomenon. I think that "Where have you been all my life?" is the most specific response to that particular situation, but it is more a rhetorical question than an expression. Sometimes people say "[they] found what [they've] been looking for", but that expression doesn't strictly apply to romantic interests, or even people.
Solution 2:
The stereotypical thing to say is "where have you been all my life?"