I am an American who has followed Brexit for a while and have a view that I suspect is wrong and would appreciate an informed correction.
From the outside it seems that Theresa May, a conservative who initially opposed Brexit, has tried to responsibly work towards implementing the results of the referendum. To me it seems that the difficulty is that many people wanted outcomes that were irreconcilable, perhaps based on different (or poor) information, anger, political views or simply self-interest. I am not aware of any argument that explains how she could have gotten a lot better deal from the EU.
On the other hand, it seems to me that many members of her cabinet and party ranged from not very competent to not very honest in their dealings with the issue. In short, she seemed to be "the only adult in the room" at times, working hard on a task that might well be impossible. However, the impression I get from British media is that Britons hold her in low regard.
I am not concerned with the arguments for or against Brexit in this question, or what is its best form. I would like to learn what, given the situation, she could have done better to resolve Brexit given that she was committed to implement the results of the referendum.