Yes, shortening of the vagina is a possible consequence of a hysterectomy, and that is clearly what you are describing. You say you were previously using an Estring vaginal ring to address menopausal symptoms; the Estring won’t work for you now, since having it inserted is uncomfortable. Oral HT isn’t an option for you because you’re a breast cancer survivor.
Please don’t lose heart. I’d recommend you consider using Intrarosa, a newer, very effective treatment for postmenopausal atrophy. It is a nightly vaginal insert, non-estrogen. Osphena is another non-estrogen oral option, or you could use a cream form of vaginal estrogen. Any of these is likely to be an effective treatment option, and all of them are considered safe for breast cancer survivors.
After restoring health to the vagina, you may then need to use vaginal dilators. Dilators are designed to increase vaginal “capacity,” whether in width or (as in your case) length. A healthy or “estrogenized” vagina should be distensible and elastic to get back the necessary length. Most women are very successful in regaining this function.