I know how you feel. I was on Tamoxifen for just over a year with few side effects and then started bleeding quite heavily. This has continued since November last year.
I had 25mm thickening and had a couple of hysteroscopies before getting a Mirena. I chose to give up Tamoxifen due to this condition. It's a personal choice though. It's good to keep asking your doctor questions. I hope everything goes well. I feel much better since stopping the Tamoxifen, although I'm not on top of this condition yet. Quality of life is my priority these days. :)