How can one sincerely repent of fornication and masturbation, while desiring to marry the girl with whom one committed fornication, lacking the financial means to marry currently, and finding it difficult to leave her?

Repentance from zina (fornication/adultery) is obligatory and immediate, as it is one of the grave major sins. If the woman was complicit, you owe her nothing, and you must part ways with her and sever the relationship. It is permissible to marry her if she repents, and poverty is not a religious impediment, for marriage can be a cause of wealth. If you are unable to marry, then you must fast, avoid provocations, associate with righteous people, and occupy your time with beneficial activities. Attention should be paid to the details of the ruling concerning 'urfi marriage (customary marriage).