I intentionally vowed by Allaah not to marry my fiancée. I also vowed that she is like my mother to me for the rest of my life. I did all these intentionally while knowing what I was doing. My question is that can I marry the same lady or she is Haraam for me. Please explain because we seem to be coming closer once again and marriage is an important issue in Islam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Yes, it is permissible for you to marry your fiancée that you mentioned in the question but you are obliged to do the following if you marry her:

To expiate for breaking the oath because you made an oath not to marry her and then you broke it, so you are obliged to expiate for breaking an oath, for details please refer to Fatwa 87181.

Your saying 'she is like my mother to me for the rest of my life', is Thihaar (claiming one's wife to be sexually impermissible like an unmarriageable woman), so if you marry her, you are not permitted to have sexual intercourse with her until you expiate for Thihaar. The expiation is mentioned in the last paragraph of Fatwa 88080, so please refer to it.

The majority of scholars are of the view that making Thihaar on a marriageable woman is acceptable and becomes applicable if he wishes to marry her and one has to expiate for it.

Allaah Knows best.