I assume that the 15 mm thickness measurement you are referring to is the thickness of the endometrium, which is the inner lining of the uterus. In a woman who was post-menopausal (e.g., had stopped menstruating completely), then a endometrial thickness of 15 mm would be something that would need to be worked up, because it would be abnormally thickened, which is sometimes a sign of endometrial cancer .
However, in someone who is still menstruating (like you are), then this thickness most likely just means that you are going to menstruate soon and, in and of itself, doesn't necessarily mean that you need workup at this time.
However, it does sound like you are having very severe perimenopausal symptoms, and that is definitely something that you need help with. I strongly recommend talking with your primary care doctor
or your OB GYN doctor about how your symptoms are impairing your quality of life. Many of the perimenopausal symptoms, such as mood swings, cramps, irregular heavy periods, and hot flashes can be controlled with medications and do not need to be "suffered through". Make an appointment to discuss these issues with your doctor at your earliest convenience.