Anyone who has a problem with what you’re saying needs to take a step back and ask themselves how they’d feel if someone did the same thing to them. Asking someone first if they would like some help is simple respect. I deal with elderly people a lot, and find that many people just assume that they want assistance, without even asking them. I think it’s very disrespectful not to allow someone to do something for themselves if they want to.

Please do not say sorry. the person needs to learn this.

Sounds odd a person trying to do a good deed and you yelling at them.
Guess you were having a bad day.
It happens , wheelchair or not.
I bet the other person feels real bad you yelled at them when they were just trying to help.
Maybe they wont help someone again.
Flip side of the coin.

And I’d also ask first but you need to expect that people will do the right thing in a public space and that is to help a person who is compromised. Pick your shit up when you drop it at home and assume others will help you pick it up out in the world. Again, I agree with what you’re saying but think of how people might feel not offering assistance. Like a garbage human. No need to bark. The fact you get annoyed at that is a telltale sign that perhaps your mental health is more of an issue than your physical incapabilities.

I stopped opening doors for strangers because it puts us inside of 2m. Same in this situation

I became disabled suddenly and the way people looked at me and treated me totally changed. The pity was even harder to cope with than the indifference. Until you've been in a wheelchair, you won't really understand.

I was having a very good day enjoying the weather until...
If ever you drop your keys or phone and just as you’re about to pick them up, out of nowhere someone who doesn’t want to feel like garbage tries to snatch them from you please tell us your reaction so we can judge the state of your mental health

I agree with the first comment one hundred percent. The second comment talking about the flip side of the coin. Never helping again is a valid reaction... although I think you will help again. I feel ya. Hurt people hurt people. But not ALL hurt people hurt people. Let’s avoid the hurt.

I grew up in a mid-size city where civility and helping each other is the norm. I quickly learned that isn't the case around here. If I see a senior lurching down a bus aisle because it suddenly brakes, or notice a mother struggling to open a door with her stroller, my instinct is to help. Yes, asking first is ideal but not possible in every situation. After encountering one too many folks like you I've learned to suppress that instinct. Don't be surprised that no one comes to your aid when you actually need that help someday.

You don't need to apologize for their bad behaviour and you are not ungrateful. What people like @Anonymous and @i appreciate this don't seem to understand is something called boundaries and that helping isn't conditional on getting something in return. No means no. Period. If hearing no means they're likely not to help anyone in the future, they're the ones with the problem! You know who I wouldn't help, people with terrible boundaries. You've got nothing to be sorry for OP.