There is absolute correlation between estrogen dominance (progesterone deficiency) and hypothyroidism. Too much estrogen not only causes the estrogen-driven cancers (breast, uterine, ovarian, testicular, prostate, etc), it also inhibits thyroid function at the cellular level. Cortisol from too much stress also has this effect. Additionally, under chronic stress and trauma (make no mistake, going through breast cancer is traumatic!), cortisol depletes progesterone, as it's actually "stolen" in order to make more cortisol. All this can be happening, and what a person has is cellular hypothyroidism, while all the labs show thyroid function as "normal" and doctors completely overlook it. I'd suggest seeing a functional doctor who understands how these relationships all work, in the hormone structures.
Bottom line: You need to know what's going on with your estrogen and progesterone levels. You need to know what's going on with your cortisol levels. How's your body temperature? Because if your body temperature is consistently low, then you are definitely having a thyroid problem on some level, and I'd bet it has to do with estrogen dominance and out-of-control cortisol levels preventing thyroid function at the cellular level. AKA cellular hypothyroidism.
Another thing to note is that unopposed estrogen makes us gain weight. Not only by expanding our fat cells, but also actually creating even more fat cells. Estrogen is also inflammatory in many cases. Now - add to the mix - fat and thyroid: the fatter we get, the less our thyroid hormones work, and the less our thyroid hormones, work, the fatter we get. More inflammation - fat cells are actually inflammatory. Now when our inflammation goes up, our immune systems get turned on, and now we're in position to start experiencing all sorts of inflammation-caused conditions, like Hashimotos and cancers and diabetes and other autoimmune disorders. All because nobody understood that 1. we didn't have enough progesterone, and 2. we didn't have adequate T3 levels. Sadly, most the doctors who we go to are either ignorant about how all this works, or they're part of a conspiracy to keep us sick so that the entire industry keeps making more and more money off our suffering. I mean, neither progesterone nor T3 is particularly profit-worthy...
Here's some additional reading for you, if you'd like to know more from the experts who know this stuff:
I hope that you feel better and get all the very best help and everything you need.