Bismillah.

An ustaz said we must  understand and be redha with the four things that have been preordained when you were in your mother's belly - on the 120th day of pregnancy when Allah grants you ruh. On that day, Allah pre-determined your:
1. Provision/rizq
2. Death
3. Luck
4. Marriage

Therefore:
Do not be broken hearted when a girl turns you down. Don't despair. To cure your headache and heartache say:
Allahumma inni auzubika minal hammi wal hazan
'I seek refuge in Allah from feeling unsettled and from sorrow.'

If you can't seem to forget the girl, put one hand on your heart or forehead and recite:
Yam hullah huma yasha'
يَمْحُواْ ٱللَّهُ مَا يَشَآءُ
'Allah effaces what He wills.'
(Qur'an Ar-Ra'd 13:39)

While reciting that phrase, make a gesture with your palm as though you are removing thoughts of her from your heart or mind.

What is most important is to see the hikmah (wisdom) behind the failed love affair and understand the meaning of verse 216 from Al-Baqarah that:
"It is possible that you dislike a thing which is good for you, and that you love a thing which is bad for you. But God knows, and you know not."