As jaynedough said, we don't diagnose although we may speculate or bring up possibilities. I don't think you really need anyone to tell you what you know. I don't see anything terribly wrong in this. I never had imaginary girlfriends but I had what I thought of as an "ideal" girlfriend even when I had real gf's. That's kind of like an imaginary gf.
You talk as if she may seem real to you. Have you thought about how this would be if a real gf came into your life?