Hopefully in your left eye you had only minimally lost peripheral vision. That is the case with many of my early glaucoma patients; we are able to arrest progression of the disease before additional peripheral vision is lost. The all important central vision is of course, in glaucoma the very last thing to be affected. It sounds like you are a LONG way from that happening.
These days, with timely and proper care it is rare for patients to lose their sight entirely from glaucoma. NTG formerly was somewhat of a more unusual diagnosis than it is today. Technology had advanced and has allowed us to obtain a much greater understanding of early damage to susceptible anatomy of the eye.
Obviously, I do not know the particulars of your case, but if you follow your doctor's instructions, chances are you will suffer no additional vision loss.
The post below yours suggests that you seek the best specialist you can find. Quite frankly, in the vast majority of cases, that is wholly unnecessary, as nearly all glaucoma cases can be efficiently and successfully managed by an optometrist or general ophthalmologist. A responsible doctor will certainly refer you for more specialized care if your case is a complicated one.
Best wishes on your future ocular health.
For all others Freepers reading this, please get screened. It takes only a few seconds, is not in anyway invasive, and is very inexpensive. It literally maps the thickness of your optic nerves and compares it to the rest of the population in your age, gender, and ethnic group. If it comes back normal, that's it, if not you then advance to other tests, and it could save your eyesight by catching it early.