As a parent, I can completely understand the frustration that this must cause. As a psychologist, I try to look at the situation putting the frustration aside and seek to answer some questions.
Without knowing more information about the situation, it’s hard to evaluate her comments about how “only big girls use potty.” She may have picked up on your language about big girls and is saying these things back to you. And I am sure it gets a reaction because it seems concerning, and you are obviously considering the possible effects as she grows older.
However, it’s possible your daughter wasn’t fully ready to be potty trained. Children who are really potty trained want to use the potty. If she is having frequent accidents and doesn’t seem concerned, she might not actually have been ready when she was 3, and then the virus and the “step back” have snowballed into a really tough situation.
In the absence of a medical condition (that a pediatrician could assess), it may be that she needs a little more time and a little less reaction from you. My sister had a very similar situation with her son and they spent hours in the bathroom, were always washing undies, and all felt pretty lousy.
Success came when my sister took a step back and re-evaluated her level of distress about the situation. We talked about how “he won’t walk down the aisle in diapers” and noticed that the less time we spent on the toileting, the more he wanted to go and the more he actually did go.
As for school, I tend to talk to all kids about their “job,” which is to be part of a family, have fun, and go to school. Modeling what’s expected (going places, meeting people, and eventually separating from mom) will be your key to helping her find success.