I think there are 2 things going on in the child's mind. One is that she's got into her mind that maths is tough or something negative when it is not. This is seen from the fact she is able to grasp quickly. The second is probably a result of the first, causing short attention span.
I would assume the 'classroom' is free from distractions/interruptions from others in the house or TV or whatever, but is conducive for teaching. If not, keep a special place and make sure others in the home do not interfere in any way and turn off the TV for that one hour or so. You the teacher will also need to keep mobile and other things aside till after class. This way the class time, the teacher and the student gets respected.
You could try out some ideas and see which works good. Keep a list of other ideas too, so what doesn't work now may work at a later stage. Hopefully doing this over a few weeks could bring positive change and progress. Remember to always have a sense of humor and upbeat atmosphere and show appreciation for any improvement.
Let her do maths for say 10 -15 minutes. Then give a short break of 2 minutes using a timer or countdown clock. Then get back for another session for 10-15 minutes and so on. What you do in the 2 minute period depends on the student but specifically away from school table:
-- It can be a quick spurt of vigorous exercise activity – dance steps or jumping jacks etc.
-- A quick walk around the house if possible but back in 120 seconds limit
-- Play with pets
-- Watch 2 minutes only of a video not seen before
-- Eat only one salted peanut, keeping a bunch for reward at the end of a certain progress achieved
You can change the time and what to do anyway to what suits the situation. I have done such things successfully with some students while teaching them. It was tough until I changed the way I taught, and chopped the day's goal into 4-5 smaller parts. Sometimes I would change without notice also, so the student didn't know what to expect but could look forward to something interesting suddenly. It provided lots of motivation for the students to keep reaching those shortened goals several times a day. The 2 minute break helped to defuse my tension too. Everybody happy and progress made.