Can I remarry him and apply for the permanent green card right now?

Generally, when a foreign national has entered the U.S. lawfully and with inspection (such as with an F1 student visa), and then becomes married to a U.S. citizen, the couple can succeed with a marriage-based adjustment of status application in the Immediate Relative visa category. This is true regardless of whether the couple may have married previously, become divorced after six months, reunited, and become married again. If neither spouse is a U.S. citizen, and instead one spouse is a Lawful Permanent Resident (has a "Green Card") and the other spouse is in valid nonimmigrant status (such as F1, H1B, etc.), it still may be possible for the couple to apply for the nonimmigrant to become a Permanent Resident, but that would require an application in the Family-sponsored Second Preference (F2A) visa category. There is a backlog for visas in that category - the October, 2017 Visa Bulletin issued by the U.S. Department of State shows that visas now are available in that category for cases filed in mid-October, 2015 or earlier. There are additional considerations and eligibility requirements to take into account too. There really is no substitute for you and your fianc? (former husband) to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.