There is nothing you can do if he is going to marry someone else. If he were in love with you, he would be marrying you. When a man is truly in love with a woman, he will do anything to be with her, and he will want to marry her. If he didn't ask you to be with him forever, he isn't really in love with you. You deserve to be with someone who will love you completely, and want to marry you. So, my advice is to move on, and let yourself fall in love with someone who is available, who will want to be with you forever.