When I was young, I took a blood oath with my girlfriend and we promised each other to never part ways, and if one breaks the oath he or she would never be happy and successful. How can I undo the oath?
Taking an oath on anything or anybody other than God is wrong. Such oaths are invalid. Narrated Abdullah Ibn ‘Umar:
“The Messenger of Allah (pbuh) found ‘Umar al-Khattab in a caravan while he was swearing by his father. So he said: Allah forbids you to swear by forefathers. If anyone swears, he must swear by Allah or keep silent.” 
“The Prophet (pbuh) said: Do not swear by your fathers, or by your mothers, or by rivals to Allah; and swear by Allah only, and swear by Allah only when you are speaking the truth.”
What you need to do is repent sincerely for saying these words, and avoid taking another oath on anything other than God. Since this is not a valid oath, you do not need to expiate for it.
 Reference: Sunan Abi Dawud 3249 ; In-book reference: Book 22, Hadith 8; English translation: Book 21, Hadith 3243
 Reference: Sunan Abi Dawud 3248; In-book reference : Book 22, Hadith 7; English translation: Book 21, Hadith 3242