You will probably have to wake up sooner or later and accept that daydreaming about some girl, without approaching her, really is pointless and you're wasting your time. The longer you do it, the more you prepare yourself to disappointment when reality slaps you in the face.
You reach a point where you've created an image of someone which is almost entirely made up in your head, who doesn't actually exist.
The fact that you don't actually know this person (much) shows that it's mainly infatuation. Maybe she appears so attractive that you tell yourself "Oh how my life would change if I could be with her. The happiness it would bring is undescribable".
You also have to accept that it's not because you "want" someone so much that you somehow understand them or deserve them more than anyone else.
If you can't accept this, you're no longer seeing the girl as an individual but more like a potential possession in an obsessive way.
I spent way too much time during my high school years, infatuated by girls I barely spoke to. It would come to the point where I would sometimes ride up to a girl's house and stand there for a while, just hoping that somehow she would come running out, happy to see me.
You're either going to learn the hard way or get over it quickly. What's more likely is that it will be the hard way.
In a way, I actually miss the infatuation I used to have when I was a bit younger. Most of the time the infatuation was with a girl I actually knew reasonably well, but whom I didn't ever know how to approach, being the big fat loser I was back then. The strong feelings that you experience are so strong and despite having a couple of relationships where I really felt close to the girl, I don't think that "desire" has ever been that powerful. The trouble is, it's often the wrong type of desire: because something is missing in your life to make you the person you want to be, you believe that everything will somehow sort itself out if you can just have that one girl.
After many wasted opportunities, disappointments, unhappy relationships, I don't think I'll ever be infatuated the same way ever. But actually it's probably a good thing.