You have already decided on some thoughtful advice, but I thought that perhaps I could offer another possible way to do things.
Disclaimer: I am in the United States (U S A); I've been to a few other places, but never Deutschland.
I myself have dealt with a recurring eye injury for some time now; I can see legibly out of one eye at the moment, but who knows about tomorrow?
Furthermore, my condition can be quite painful. The blurry vision is less of a problem than the pain with any attempt to use my eyes when the condition is at its worst.
Anyways. One of the things I've learned is that such conditions are only impairments when we compare them to the conditions of others. At the heart of the matter, they are merely a special set of parameters with which a person must learn to cope. Everyone does that; sometimes they take for granted a given condition, like the ability to read at 20/20 vision.
That's where the problem arises, I think. Think of it another way: almost nobody misses the inability to detect the direction of odors or the inability to hear a field mouse scratching at dirt from 20 meters distant. That is because they never noticed the difference between that and their usual condition.
It is much the same with lack of vision or mobility. Someone who has never experienced or even contemplated such conditions cannot possibly conceive of such a life; sure, adjusting to the difference can be difficult, and require a bit of help at the outset. Once you learn the tricks, though, it isn't much different from how most people live their lives: for example, most of us are unable to directly perceive the ubiquitous cosmic rays that pass through our bodies every hour of every day, for example; as another example, many people nowadays completely dismiss the possibility o unseen, larger dimensions to our universe which has been described in the past as some variation of a spirit world — if you believe in such things.
How I do digress and ramble along.
One possible choice of actions that you could consider is to have a simple and convenient explanation ready for people who insist or seem obligated to treat you as if your blindness cripples you, or renders you invalid. I would not recommend going through everything I've spewed in these paragraphs, of course.
Here are some examples:
- Is it because of my blindness? Really, it isn't a problem for me. I've learned to live like this, and I am happy and doing well.
- I appreciate your compassion, but you don't need to feel sorry for me. Indeed, not being able to see has really opened my eyes to ways of experiencing the world around me in ways that most people go their whole lives and never know.
- I respect the gesture of goodwill, but I honestly don't need it. I don't even consider my lack of vision an injury. I'm no more disabled than you or anyone else; I simply deal with things differently.
- You know, I voluntarily exchanged my eyesight for superhuman enhancements to my other senses. I can wrestle you to the ground for a demonstration if you would like.
(That one was a bad joke. Don't judge me too harshly, please.)
Four examples. That should be enough to convey the gist.
I strove to make the examples concise and varied. I hope they don't read as being too hackneyed or cliché.
Does this all sound too melodramatic? If so, then I hope it can be useful or helpful despite all that.
In conclusion, it all has to do with your demeanor: pace and tone of voice — as you well know. Take care not to give the other person a reason to feel sorrow for you. If you've had a long and trying day, then you can shorten your responses, of course.
The thing to remember is that there is a chance the person with whom you are interacting genuinely doesn't understand what things are like for you. They probably aren't meaning to make you uncomfortable: they simply do not know any better. That is the whole premise behind my recommendations: education, one person at a time. Turn it around and be the one helping them.
If at the end of all that, this short essay is not helpful, I would like to quote something that someone once said: