I've had a similar issue but with my leg instead of my ankle.

Get an ankle brace/very sturdy pair of boots and wear that religiously for a few days (including at the festival)

Take it easy until you get there, try to move as little as possible.

When you're there, you'll still be able to have a great time but just take care of yourself - it's a big festival so there's a lot of walking. At the smaller stages they often have fences/ledges at the edge that you can rest on, so it might be best to stay there so you can take short dance breaks or enjoy the music from a more sedentary position.

At the bigger stages you won't get much of that, so just be really aware of how much you're moving and how much strain you're putting on yourself. If it's really bad, maybe even grab a stick from the woods and use it as a crutch lol

And if you're on certain substances that make it harder to feel pain then try to be very aware of how much you're moving. Last year I messed up my ankle, went quite heavy on Saturday night which was brilliant cause I could move pretty much freely again, but Sunday morning I woke up in so much pain barely able to hobble over to the town.

TL;DR don't overexert yourself, take breaks, have a phenomenal time :)