What if I'm already on Zoloft (sertraline) & have been for years (50 mg) but now it doesn't seem to be helping at all & I am seeing that I must be very depressed.?

Need change. After your doctor decides nothing else is causing recurrence of depression, a change in dose might be useful. You definitely want to let your doctor know about this so that he/she can help you.
DOSAGE SUPER LOW!!! I would speak with your physician because the dose you are on is super low so there is a lot or room for adjusting and augmentation with other meds. Have to see why there is a dip in mood after all these years as well so really need to speak to your doctor.
Zoloft (sertraline) Talk to you doctor about possibly adjusting the low dose or other add on alternatives.