That's why you would stalk some girl from that time who knows you. (source - I've been there.)
But why one you haven't met who doesn't know you? If it's pure fantasy, you don't have to contact her.
Also my therapist says "fantasy existence" is only bad when it has priority over reality and one's real wishes. Also "developmental goals" can be achieved in various ways, and some are better abandoned earlier rather than later. Being very stubborn you might crack some, feel as if reaching a mountain top for a week, and then get depressed.