By mistaken, if we stay in haram relationship, if we did love with someone and if we want to be and stay connected to Allah but often realize that haram sin I have commited; so can I leave him leaving a commitment to marry him only in future. If Allah wills….

Yes you can tell him: I don’t want to commit haram. In the future, I am interested in marrying you. Don’t give a promise of 100% because you might change your mind. Just say that you plan to and you would be interested in him only.