An old filling is on a nerve and the dentist is recommending a root canal since i'm having sensitivity with hot and cold. is that my only option, what would happen if i left it? i most likely won't be able to have root canal for a couple months?

What will most probably happen is that the pain will worsen so that it is sensitive all the time, and then the pain will increase to the point where it is unbearable. How long this will take is uncertain, but what is certain is the need for root canal therapy. The sooner you treat the tooth the sooner you will feel better.

You will have more discomfort and probably pain until you get a root canal. A pulpotomy could be done until the root canal is performed or you can live on pain medicine or not eat on that side. You don't want to lose the tooth so get some kind of treatment