Allaah does not enjoin something that is harmful for His slaves, and the Laws of Allaah are based on wisdom and the rulings of sharee’ah are meant to be easy. Allaah does not burden any soul beyond its scope. If something in Islam is too difficult for a person, then there should be a way to make it easier, as is the basic principle of sharee’ah. So if a person is unable to pray standing up, he may pray sitting down, and if a person is unable to fast – because of old age or chronic sickness – then he does not fast and he should feed the poor instead, and so on.
Women still wear the niqaab (face veil) and khimaar (headcover) and we do not see any of them complaining about it or finding it too difficult. Perhaps what has happened to our sister – or others – is because of the quality of her niqaab or khimaar, or because of the way it is worn. If a woman is wearing a thick niqaab then that may make it difficult for her to breathe or to see, so the solution in that case is to make it thinner.
Some women tie the niqaab very tightly over their faces, which makes it hard for them to breathe. The solution in that case is to loosen the niqaab so that it will be easy to breathe.
It would be a good idea for you to refer to a doctor who has more experience than the doctor you are currently seeing, and perhaps they will be able to find a solution to your breathing problem that she has not come up with. If it is not possible to solve this problem by any means – which is unlikely, in sha Allaah – then do only what is required to ward off harm in this case. Do not uncover the whole face for the sake of breathing; you could put part of the niqaab lightly over the nostrils as much as you need to make it easy to breathe, even if that is less that what is ordinarily required.
We ask Allaah to protect you from all evil and to bless you with righteous offspring. Ameen.
And Allaah knows best.