I have a girlfriend and we promised not to leave each other. But I came to know that these are haram relations. So can I leave her and break my promise and her heart. Can I be forgiven?
If you want halal relationship with her then marry her with Nikah. Else it is wajib to leave her since it will lead you to zina. Promise to do sin is not valid.
Assalamualaikum. It is stated in many Qur'an verse that Allah.SWT is merciful for his creations.As from your conditions,its is OK to leave your girlfriend (it is actually suggested to leave her).Since you didn't know it was haram,Allah.Swt will forgive your sins inshaallah.Try to tell her why you are leaving him and try not to break her heart as it might upsets her.
If you are not serious about dating her into marriage,i suppose you must break up.Unless if you want to keep the promise of never leaving her.I dont really know about the facts that if its ok to date to marriage,you can search it on google/ ask the experts. Well then,Wassalamualaikum.