Your concern about your myopia is ill advised. The most likely scenario by far ( greater than 90%) is that you will have good vision all your life. As a high myopia you have somewhat higher than normal rates of retinal detachment, glaucoma, macular degeneration but in an absolute sense these are small. e.g. instead of the regular risk of retinal detachment 1 in 8,000=12,000 your risk might be one in 5,000. More likely anxiety is a much bigger problem than the high myopia. Most people with general anxiety disorder are always worrying about something. I would suggest you wander over to the anxiety and depression forums.
From a non-medical eye professional and from someone who has traveled twice as far as you have in life, I would like to reassure you. I have had really high myopia all of my life. When I was in elementary school, I was at -5 to -6. I have been at -9 (in glasses) and -8 (in contacts) since your age. I am now age 56. I guess vision stabilizes at some point. I have had PVDs in both eyes with no retinal detachment. In fact, both PVDs went smoothly and quickly (although I now have two Weiss ring floaters to keep me company). Because of my high myopia, the PVDs occurred earlier than it would for "normal" people. The first was at age 45, and the second was at age 52.
I also understand the anxiety. I get it over alot of health issues although I never worried about vision at your age. Losing your vision is a scary thing. If I could predict your future, then I would say that you may go to a -7 at most. You will get reading glasses a little bit earlier than some, but it doesn't mean you will have poorer vision. I use +2 for reading glasses, and it is the same as my wife's who hardly needs glasses for distance. You may have PVDs in your eyes earlier than others, but this does not mean you will have much more risks than they have. You may be stuck with the Weiss ring floaters because of your higher myopia, but if or rather when you accept them in your life, then they will not be much of a bother.
I cannot comment on AMD or glaucoma or cataracts. SO far, I have none of them. Pretty certain since glaucoma is not in my family, then it means I am a a low risk for it. I am concerned about AMD, but again, no one in my family has it. Not sure if that means I am at a lower risk there too. It is one of the questions I will be asking the experts when I get a chance.
Relax. Enjoy the life you have. Your future vision will most likely not be much different than what it is now.