What should I do about my desire to marry a girl whom I fear another might propose to, while my parents refuse my marriage now?

If you are able to marry and fear falling into the unlawful (haram), and your parents refuse your marriage, then you are not obliged to obey them. Instead, gently present your opinion and remind them of the Prophet’s (peace be upon him) hadith: "O young people, whoever among you can afford to marry, let him do so, for it helps him lower his gaze and guard his chastity. And whoever is not able to do so, let him fast, for it will be a shield for him." And explain to them your need for marriage.