I must stress I cannot give you advice and you must discuss diet with your medical advisor, and even more so when in treatment.
There is a significant body of evidence linking deficiency in long chain omega three (found in oily fish / fish oil) and depression. At the most basic level the brain does not work properly without adequate DHA. To save me prattlling on this is a good book that explains. "Bookfinders" is another way to get a copy.
http://www.amazon.co.uk/Brain-Buildi...3159605&sr=1-1
And if you search under omega three, fish oil, Greek diet, breast cancer diet, etc on this site you will find there are quite a lots of indications that the omega threes may reduce risk of BC, improve some treatment outcomes etc.
Herceptin works as one of its legs in the fats pathways. I have asked on many occasions if it blocks the body's ability to convert mother omega three or six or both to the long chain children which includes DHA. I have seen no answer
IF you cannot make DHA from the mother fat, if you do not have supplies of the mother omega three, or do not eat it in your diet, you do get it. The body cannot make the omega three and six mother fats.
Because most eat a lot of high six low three vegetable oils, and we feed our animals food high omega six grains most have far more omega six than three.
SO for most the issue is omega three supplementation (and looking at cutting intake of omega six to bring into balance with omega three intake)
If you do find this an approach you might want to explore further please ensure you discuss dietary change with your doctor. Fish oils can cause blood thinning.
Equally you should consider discussing your depression with you medical advisor.
I am very sorry to hear what you are going through, and wish you hope and a brighter day.
RB