Don't put yourself into the trap of loving only one person
forever. Once you know it's gone case, just move on, new
target...else you are going to be trapped into this girl forever.
You are missing out how many great girls there are outside. Every
girl is special in her own ways. Every girl is
cute/pretty/special/unique/whatever.
How long do you know her? I don't really suggest getting a gift of
100 bucks unless you are damn close to her. If you are not that
close to her, a simple gift of less than 50 bucks will suffice. It
has to be something that stands out and something that can remind
her of you. Don't get something too cheap though. Do attach a card
with creative drawings of yours but no mushy stuff. You don't want
to scare her off on her birthday.