Capegirl is completely right-it may take time to find the proper treatment for you; but once found you do have the possibility of living a depression free life.
Depends a lot on what treatment you're getting. Have you had both cognitive-behavioural therapy from a trained psychologist and anti-depressants? How many different anti-depressants have you tried? Have you tried mood stabilisers also? Has bipolar been definitely ruled out? Have they suggested electro-convulsive therapy, which can be very effective in treatment-resistant depression? Untreated or not treated properly, it may well keep coming back.
Please, try not to think about giving up until absolutely all the treatment options have been tried. Hopelessness itself is a symptom, not truth.
It does seem to go up and down, i've been down to the bottom and back many times. I am finding the best hope so far for me is using the Power of Now by Ekhardt Tolle. To retrain your self talk seems at least for me, to change my outlook, i now have hope.