There is no doubt that relationships between men and women who are not their mahrams (close relatives who one can never marry) are haram (unlawful). This has been discussed previously in the answer to many questions, including 93450 and 26890. We would like to remind you of the seriousness of this relationship, in response to what you say in your question: “I have been having a boy girl friend relationship with a girl for 6 years”. We praise Allah for having blessed you with guidance and enabling you to become religiously committed and to repent.
We have previously explained on our website who is the religiously committed woman who a man should look for when he wants to get married. See the answer to question no. 96584
It is on this basis that the wise man should choose the one who will accompany him in this world. It was narrated from ‘Abdullah ibn ‘Amr that the Messenger of Allah (blessings and peace of Allah be upon him) said: “This world is temporary joys, and the best temporary joy of this world is a righteous wife.” Narrated by Muslim, 1467; Ibn Maajah, 1855, with the wording, “Indeed, this world is no more than temporary joys, and there is no temporary joy of this world that is better than a righteous wife.”
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Women may be married for four things: their wealth, their lineage, their beauty and their religious commitment. Choose the one who is religiously-committed, may your hands be rubbed with dust (i.e., may you prosper).”
Narrated by al-Bukhaari, 4802; Muslim, 1466.
The essential matter in the nature of your relationship with this girl and her family is that they know that you are religiously committed and are keen to follow Islam and the Sunnah, whether with regard to your outward appearance or your character and interactions, or your wife and household. So they have to understand that the foundation on which your life is based is Islam, and that they have no right to try to have a say in any of that, or to interfere in your private matters, especially those that have to do with religious commitment and the Sunnah.
In that case, if her father refuses to let her marry youfor the reason you mention, then you should not worry about him or about the family at all, and you should forget about this girl and look for a wife from another family who will accept you and the path you are following, and will be in harmony with you instead of asking you to compromise in order for them to accept you.
The same applies with regard to the girl. Even though you have told us of some of her other good characteristics, the basic problem is what you yourself said, that she is a very humble person and cares about her parents very much, to such an extent that she refuses to wear hijab or Islamic clothing before marriage. Yes, if you felt that she dislikes her family's opinions and that she is keen to become as religiously committed as you are, but she is not able at present to go against her family, but if she marries you she will be obedient to you and will follow your wishes with regard to wearing hijab and following the Sunnah, then we would say yes, try to marry her and take her out of the environment which is not helping her to obey Allah.
But if she gives into her family’s opinions then we do not advise you to marry her at all, because this is something that may cause many problems between you later on, and it may be difficult for you to continue married life together, if the situation is as you have described.
To sum up:
We do not advise you to go ahead and marry the girl you mentioned, because it is fraught with serious matters that may affect the very foundations of the marriage.
Yes, if they accept you as you are, without stipulating any conditions that have to do with how you live your life and the choices you make for yourself, or interfering in your private affairs, and you know that the girl will help you in that, then in this case only we would tell you that there is nothing wrong with marrying her.
We ask Allah to guide you and bless you with a righteous wife.
And Allah knows best.