Are you talking to a counselor or therapist about this? If you are, as you say, codependent, you may need help to break the ties that bind. There are also support groups out there that can help you learn coping strategies and give you a place to vent.
Meantime, may I suggest screening your calls? Seriously, when she calls, DON’T ANSWER. Let it go to the answering machine. Turn off the ringer if you have to. Hell, take a complete vacation from contact with anyone at all for a weekend. I do this from time to time; it’s amazingly therapeutic to thumb my nose at the phone and say, “Nyaah, nyaah! You can’t make me!”
Tell her you so admired her choice of a place to live! As you inferred, from it’s inconvenient distance from you, that it was her attempt to insure she did not become dependent on you, and was actively striving to maximize her own independence.
Then tell her, with lead time, you are happy to help her, from time to time, but clearly you cannot be expected to managed her daily/weekly things, doe to the distance, and your own busy life in the city.
I would stick to this story like it was glue.
I would do a little research, make some calls, local service organizations, (the elks, rotarians), churches, town hall, inquire about resources for someone blind, new to the community, etc. You may well find local contacts that will willingly step in to help her.
What is clear to me is that unless your mother changes her behavior, she will eventually die of this disease. You’d think losing your eyesight would be incentive to lose weight and take care of herself, but apparently not.
What is also clear to me is that unless you change your behavior, your relationship will remain the same. Like children whose parents still do their kids’ laundry and cooking even though they are adults and supposed to be able to care for themselves, unless you force your mother to do something else (e.g. get groceries delivered), she’ll continue to take advantage of you. For her sake, you need to let her take care of herself.
How about giving her a set amount of time that you are able to help each week? That way you won’t have to worry about saying the n word and she will know the limit.
Say “Mom, I love you but I also have to take care of me so I don’t end up like you, and you have to start learning how to take care of you. I can spend two hours a week helping you with whatever you want. I can’t take any more time off of work because not being paid means I get behind on my payment plan.”