Regime for adjuvant therapy

I am now 45 years old, exactly 3 years ago I did modified radical mastectomy for 2cm.size invasive ductal carcinoma with oestrogen negative receptors and 20%sensitivity for progesterone.All axillary lymph nodes were negative for malignancy , C53 protein was negative.I didn't take chemotherapy nor radiotherapy after surgery,I am taking zoladex every month plus tamoxifen 20mg daily since 3years. I was menstruating before starting these treatment.Now, my question is: for how long should I continue to take zoladex, and should I shift to new generations of aromatase inhibitors instead of tamoxifen like aromasine(although it is written that these drugs should only given for postmenopausal women).In summary, please prescribe clearly for me the best regime for the next priod to prevent recurrence of the tumor.Thanks alot