Longish reply here. Some of the things you wrote feel familiar from my past, so I can relate a little even if circumstances are different. I don't know you or psychology, so my ideas may be all wrong:
>such a deep addiction to something so stupid
It's nothing to feel ashamed about. Internet is
naturally addictive, especially interactive feedback loops like Facecancer and imageboards, and especially for people who have no superior stimuli in their lives.
Perhaps it's a consolation to you that you may have wasted your prime years, and any topical knowledge you've accrued may never be useful, but you know there are way worse addictions, like substance abuse or gambling. I'm just saying this so you can hopefully feel a little better by comparing your situation to those people who've had it even worse, and appreciate that your addiction is less destructive on body and wallet.
More importantly, wow you're still young! Don't think: "How can someone so young already be such a failure?" I think your young age is an asset and greatly improves your chances of somehow managing to pull yourself out of your situation. Perhaps not soon, perhaps not on your next try, perhaps slowly, just keep trying.
How? I don't know. The only bit of advice I can think of is, when an opportunity arises, arrange for a couple of days (ideally, a holiday trip) during which you think you can get enough alternative stimuli to stay sane, so the cravings can weaken during this time. So, instead of only shutting yourself out of the internet, you should make sure to first find something else to occupy yourself with, just long enough and just interesting enough to allow the brain to get off the internet "drug". The more novel and immersive, the better. What will work best for you probably depends on how much of an introvert or extrovert you are (by character, not by current lifestyle) and what is and isn't novel to you. Since you cannot travel to an exotic location any time soon, and assuming you have no friends, perhaps you'd enjoy trying a handcraft, learning an instrument, dancing, working out, writing anything, or even just reading books or mangoes? In spring, hanging out in nature reserves? In summer, taking a swim? If you feel daring, looking for people with a common interest and talking to them? Preferably, anything without a moving (or updating) screen. Best thing would of course be off-screen interaction with friendly humans you like, but I doubt you have access to that. Animal interaction can make people happy, too, or even just watching them. Visit wild animals, ask if you can play with a dog. So, for some days or weeks, limit whatever habit you want to shake off as far as circumstances permit. If, after some time, you feel that the cravings chemistry in your brain has somewhat subsided, congratulations, hopefully you can now use the web in a moderate, healthier manner when you need to. Your thoughts may still gravitate to it, but your brain should now be more aware of alternative fun things to do or feel that you've done or felt recently. If the cravings don't subside, switch to a different replacement activity or call it a truce and try again at the next opportunity, you're still young. And you live in Germany, so you're entitled to professional assistance covered by your health insurance. Do you know that Swede who got a "state-mandated gf"? I've heard of something like that existing in Germany, for example for autistic children.