As a suggestion... look into her friends. Not as objects of sex, but as friends.

Sublimation can help you move beyond what is imminently dangerous to a life-long relationship. Finding someone with her qualities, but traits of her own that are endearing and exciting... it's good therapy.

If you ever feel impulses to act on your feelings there, definitely seek counseling, if not for yourself than for her sake. SUch a thing might be reciprocated, but your giving up a million things down the road that obsession makes unclear: her wedding, her 21st birthday...

Love her enough not to act on those feelings... get help if obsession gets dangerous.