I'm so sorry you're dealing with this.
I have no advice to offer but hope the side effects of your treatment pass as quickly as possible and that it is effective.
I have so much admiration for those living with secondary breast cancer and was a huge fan of the late Kris Hallenga.
Stay strong and be kind to yourself.
Personally I don't think antidepressants are particularly helpful when the psychological distress results from having a physical illness which has done a number on your quality of life - feeling sadness, grief, fear, anger etc isn't a mental illness in that situation, it's a normal human response to suffering.
However they may have some mileage in relieving hot flushes, and I don't know if nausea and vomiting is an issue for you with chemo, but I had this in the first weeks and months after my brain injury and found mirtazapine extremely helpful where all human licenced antiemetics had failed.