Betta fish is sick. What else can I do?

A week ago my family and I left for a week vacation so I left all my animals at my grandmothers. Im always very skeptical to leave them down thee because she is bad at taking care of things and overfeeds people and animals. I told her exactly how to feed my betta fish and found out that later in the week she decided to not feed it 2 pellets a day like i said but about 20 a day instead. A few days before we were to come home she called to say my fish wasnt eating anymore. I figured it was full but when i came home yesterday and saw her it was bad. The water was severly cantaminated with about a hundred pellets that were rotting in her 1 gallon tank. It was discusting. I was even more surpisd to see my poor fish. She has a pellet-sized growth under her mouth on her chin. She has her fins pulled tightly to her body and she hasnt ate for days. I rushed her home and completley changed her tank and added 50 drops of BettaFix (the bottle said 18drops per quart and there about 3-4 quarts in her tank). I did this last night and today she is still very sick. I understand it wont work right away but im worried because i dont know when the last time she ate. I figure that the thing on her chin is some sort of fungus from the rotting food that contaminated her water and i also figure this is why she is sick. When will she eat? I have no idea how long she has been sick, its probably been around 4-5days but i had no idea until yesterday. Is there any hope for her? She also stays at the top in the corner and doesnt move much and is always gasping for air. I hate to see my little Bubbles like this :( normally she swims constantly and her fins are up and she eats like a pig. Ive had her for months and never had a problem, so please dont tell me its because her tank is small. Is there anything else i can do for her??