What should I do if the month of Ramadan is approaching and I have not made up the missed fasts that are due on me?

You must make up for the remaining fasts you owe in Sha'ban. If you are unable to make them up before Ramadan, then you must make them up after Ramadan. You must also feed a poor person for each day you delayed making up the fast, unless you were unaware of the impermissibility of delaying the make-up fast, in which case the expiation is not obligatory upon you.