I don't know where you are but I get my treatment at Dana Farber Institute in Boston, MA.
I had a hard time first on AIs and then on Tamoxifen at 20 mg. In my case, it affected my mood. My oncologist started me at 5 mg and slowly inched up. Fortunately with the adjustment of some other medications I already took I was able to slowly inch up over a period of two years. I am now on 20 mg. My oncologist was practically dancing in the hall when he learned I had finally been able to tolerate the full dose.
I don't know this, but I think it depends more on your oncologist not on the center. I was getting treatment in Rhode Island and my oncologist there prescribed 5mg referring to the "Baby Tam" study. This study wasn't of women with the type of breast cancer I have. Mine was Invasive Ductal Carcinoma. Have you tried a second opinion with another oncologist? I know how frustrating it is to want to be compliant and your body won't cooperate and at the same sime you're scared about getting cancer again. Best of luck to you.