I'm not sure who told you that zoloft caused your breast cancer but it's important that you get your information from appropriate studies. I've included a reasonable study (ie several thousabd people) that looked at antidepressants and cancer risks. It came up no link.
Many things we read are opinions or anecdotal ideas of what people believe caused bc as they don't understand why. Most of us don't ubderstand why. Many many of us don't fit the risk factors for bc.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720762/There has been some small studies on stress causing cancer but so far no definitive results other that they found getting cancer certainly causes stress.
Stress long term can cause high blood pressure, ulcers and obesity, especially if poor sleep and cardiac disease, . Long term stress leads to depression also. So many reasons to tackle that I think.
Antidepressants take around 4 weeks to feel the benefits, which is just to rebalance brain chemistry. It can take longer though to get doses right. If anxiety is also a symptom it usually takes 6 to 8 weeks to aide this. Mirtazipine is also very helpful with sleeping difficulties but only at the lower doses. However for most people once depression /anxiety improve so does sleep.
Exercise in addition, just 30 mins a day of walking, really assists depression and improve sleep.
For full benefit with depression most people would need at least 6 to 12 months of treatment before considering a reduction or to come off. It usually is then a few months after until you will know if all okay after stopping as the brain chemistry rebalances again.
Not all antidepressants work the same on people and therefore a different sort may be needed if no improvement. We are all unique.
Hope this helps. Kath x