First of all, I'm not a medical professional so please - seek professional advice. Don't JUST take my word for it.
As far as I'm aware (and I believe I'm corect) former sufferers of breast cancer are three times more likely to have the cancer return. That is the most likely reason for your doctors diagnosis/choice of medication.
Zoloft is an anti-depressant (I'm assuming you already know) and I'm assuming it's working on your emotional issues.
I'm unsure as to the safety of bio-identical hormones re breast cancer - though I believe as a stand alone form of treatment, it is viewed as the safer option overall.
Unfortunately the FDA appear to be stalling re it's current status. I'm unsure as to why.
Try this following link - it may or may not be of help:
Let me know if you need any further help. Just bear in mind that your doctor has made the right choice according to your medical history.