I have severe nearsightedness (-7 diopter) and I wanted to know what the cause i!

I have severe nearsightedness (-7 diopter) and I wanted to know what the cause is primarily?

Is it because of the lense inside my eye is defective??
or the muscles that adjust the lense have something wrong with them?
or my cornea is shaped different (too steep or whatever)?
or is it because my eyeball is too long so the image is unable to focus on the retina?
or could it be either of these or a combination...
I do know that the cornea and the lense work together to focus the image on the retina and the muscles work to fatten or elongate the lense in order to focus the image.
I have been researching this a lot but haven't been able to find the answer so if anyone knows I will greatly appreciate an answer!!
Also, if you have an tips on how to prevent my vision from getting worse, let me know.

It isn't possible to know the exact cause for you without measuring your individual eyes with a variety of tests. Generally, myopia is due to a mismatch between the refractive (light bending) elements of the eye and the total length of the eye.

The refractive elements include the front and back curves of the cornea, its thickness and density and the front and back curves of the lens, its thickness and density. The total length of the eye consists of the corneal thickness, anterior chamber depth, lens thickness and posterior chamber depth. If the total power of the eye has a focal length that is shorter than the total length of the eye, you are myopic.

You could have tests done to measure these various elements and then compare the results to "average" eyes and possibly determine if any one element is more unaverage than the others and thus the cause of your myopia.

As the size and shape of these various elements are determined by your individual genetic make up, there is no way to prevent your myopia from increasing. Most myopes stop getting worse between the ages of 16 and 25.