Hi Manizheh: Unfortunately, weight gain is a common side effect of tamoxifen. And this gain is most likely to appear right around your middle, in your belly area. Did you go into menopause due to the chemo? If you did, and you're no longer having periods, then that's another reason you've acquired some belly fat. At any rate, there's not much you can do besides what you're doing: watch what you eat, and exercise. Don't give up; once you stop the tamoxifen, the weight should be easier to get off.
As for your fear - it's hard to shake. Please read our post on fear; perhaps it'll help you deal with it. Good luck with your Ph.D! PJH