You don't say how large she is, so that would make a huge difference in what else can go in the tank.
If you're looking for a larger tank, try Craigslist: community/pets. Look for ads, or place one of your own. You can also try a resale shop such as Salvation Army, since sometimes people turn fish tanks in.
Another option is to advertise the fish on Craigslist. Be careful that anyone who takes her isn't going to just put her in an outside pond that isn't lart enough for overwintering. Or, do they have an option for the winter?
Be sure you aren't changing the water too often. This sometimes shocks fish, so they don't act themselves. Btw, mollies and goldfish have entirely different temperature requirements.
Get a couple added goldfish, and only change 10-20% of the water, once per week. Of course, you need a filter.