Translation:Who is the girl you are always thinking about?
What is wrong with translating the sentence as: Who is the girl that you still think about?
I THINK your version might use "ješte" rather than "stále." The latter, from what I understand, has more of an ongoing feel about it, like continuously, continually, all the time. Disclaimer: I'm learning, too, so someone else may give you a more authoritative answer!