For someone your age, high myopia at the level you have just means you need to wear stronger lenses to correct your vision. I had vision that was a bit worse than yours for thirty years and it just meant I had to wear glasses or contacts all the time. I found contacts gave me better vision. Usually in your early to mid twenties your prescription will stabilize as your eyes stop growing (the exact age varies, perhaps yours might already be stable, and at your age your prescription probably won't change too much). In rare cases people's eyes keep getting more myopic even after that age range, but usually the level of myopia that might indicate "pathological myopia" where someone's vision keeps getting worse is a few diopters worse than where you are so it seems unlikely you'll get there.
When you are over 40 or so being highly myopic will slightly increase the risk of rare things like retinal detachment or some other eye conditions, but the risks are still low and thats not for a couple of decades or so (and by then the doctors may be able to prevent those problems from happening, or treat them easily)
It can sometimes take a little time to get used to a new prescription, or perhaps the prescription is a little off so you could consult the eye doctor to check on it. Eye strain when spending many hours reading is a common temporary issue that can happen to people at any age, if you search you see recommendations to give your eyes a break each hour by looking up at something further away.
Issues of light sensitivity, called photophobia are separate from whether someone is myopic. I don't know if you are mostly talking about sunlight and perhaps need to wear sunglasses more, everyone is different in terms of how much bright light bothers them. If you spend lots of time reading indoors you just may be less used to brighter lights. Whether the sensitivity indicates a problem depends on how severe it is, whether you get headaches or need to wear sunglasses even indoors or something. Its likely there is nothing to be concerned about, but if it is bad enough that you are worrying about it then you should talk it through with an eye doctor who can examine your eyes and see if there might be a problem, since there are multiple things that can cause light sensitivity and so we can't guess at what the problem might be.