We can only do so much for advice, because we're not seeing the full picture of what your horse is doing.
If your horse is just falling into the circle, you need to use more outside rein and inside leg. The horse should be bending around that inside leg. You need to make sure you are straight and not leaning into the turn.
If the horse's rear is falling out (like a car fishtailing), then what DanielDauphin suggested is spot on.
If she's drifting, it's because you're allowing her to. You need to use your aids to keep her as straight as possible between your aids. If she's a wiggly horse, it will be tough because it may mean that YOU have to be uneven to keep her even (for example if her butt tends to swing to one side then you always have to be pushing it back over to keep her straight). If she's doing it on the lunge, she's invading your space and being lazy and should be chased right back out to the edge of the circle.
If you're able to get lessons, even if it's just once a month, it will help you. Ask for homework in between sessions.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.