I am missing a girl and I cant able to know I love her or not but dont want to forget her?

It seems to me that you might be developing feelings for this girl, whether romantic or platonic. The best way to go about not forgetting people is living in the happy memories that you two may have shared in the past (or things that can happen), but beware this may also be bitter in the end if you can't ever talk to her again. If you can, though, I would try and keep in contact with her and see where that leads. I hope this helps!