While I'm sympathetic to your medical situation, I challenge the use of your word, "monopoly," in this example. A monopoly seems to be used in a derogatory way, as if the pharma company has manipulated the business environment in such a way that no other competitor can step in.
Aside from that, I'll answer your question. A patent does have a limited lifespan. In the case of pharmaceuticals, the patent may be extended when there are new treatment indications, or novel ways of applying the patented product. The reason for this is, a pharma company might find other uses for the drug through its R&D. Otherwise, it cannot get FDA approval to sell the drug for any other indication.
Having said that, once a pharmaceutical goes off patent, other generic companies may manufacture and sell that product (assuming FDA approval to do so). However, depending on the technolgy and costs associated with manufacturing, a generic may prefer to skip the more costly products; i.e., if it costs them a lot to make, they have to charge more, and be uncompetitive. You see this with a lot of the insulins, for example. It takes a long time in days, and uses expensive manufacturing techniques and equipment.
I hope you find a cheaper alternative. It sounds like a lot of good pharma R&D is working for you.
While I'm sympathetic to your medical situation, I challenge the use of your word, "monopoly," in this example. A monopoly seems to be used in a derogatory way, as if the pharma company has manipulated the business environment in such a way that no other competitor can step in.