You are fine. You don't have a degenerative disease. I have had glasses since I was 9, and believe me, the prescription got stronger every year for a long time. I think it got fairly stable in my early 20's, but even then it would change a little here and there.
As others have said, it stayed pretty stable in adulthood, but then I had to get progressive lenses at age 46 because reading got harder. You know, the old 'hold the paper farther from your face because it is blurry up close' thing that hits practically everyone in their 40's. It's a middle age thing. Obviously you don't have to worry about that yet :)
I have nearsightedness and astigmatism. In spite of this, my doctor says my eyes are fine and perfectly healthy.
Please bring your concerns up to your eye doctor. I am sure he/she will tell you that what you are experiencing is totally normal. I bet your eyesight isn't even all that bad, if you didn't need glasses until you were 13.