Why is my fish struggling to swim?

Your fish appears to be alert and active. But you say that she keeps getting sick and with the same fungus, it could be you have not treated it completely or a lot of times when fish get sick they can get damaged by it. I had a betta that got sick and he was paralyzed on his right side and could only swim with his left fins. it was sad. Your fish could have just some affects because of the diseases its been getting. I would continue to treat the fish, but instead of putting it with other fish putting it in it's own ten gallon tank, and letting it recover in there and raising the temperature always helps a sick fish.