I am sooo sympathetic to you! I am so, so sorry you are going through this. Depression runs in my family, and I had terrible postpartum depression too.
I would suggest discussing all of your options with your doctor. Pregnant women can take depression meds if necessary, so I imagine that nursing women can, too. (I know you said that you didn't want to stop nursing, but you can if taking meds and nursing makes you nervous--your baby is 19 months, after all, but if nursing makes you feel good I understand why you wouldn't want to stop at this time).
I personally would not be comfortable taking "natural" meds without talking to my doctor about it. Herbs are still drugs!! It's why you can't even drink herbal teas when pregnant! "Natural" meds aren't necessarily regulated, either, so there's no real way to know what the dosages and strengths are among brands, or what kinds of studies have been done on the herbs.
If you can see a therapist at this time, I would. It's so helpful to talk to an unbiased professional.
Just remember, a happy mommy makes for a happy baby. Sometimes we moms forget to take care of ourselves.
I wish you the best!!