How can I get rid of my attachment to my ex-boyfriend and dedicate myself to my current fiancé?

Islamic law does not permit relationships between young men and women outside the framework of marriage. The prescribed way is for a man to propose to a woman through her legal guardian. You must remove your attachment to this young man and sever all ties with him, avoid thinking about him, occupy yourself with what benefits you in your religious and worldly life, strengthen your connection with Allah, be diligent in learning matters of the religion, choose righteous company, and engage frequently in remembrance of Allah (dhikr) and supplication (du'a). The suitor is a stranger to his fiancée as long as the marriage contract has not been concluded.