See yourself out
A (mostly) sure-fire way to get out of a forcecage on your own, is to shapechange into a beholder†. You now have a pointable antimagic field spouting from your main eye, however since it's a cone shape and only suppresses the forecage in its area, you'll have a hard time squeezing out. What you'll actually want to do is starting shooting at with your newfangled eye rays, until you get a disintegration ray on it.
If the target is a Large or smaller nonmagical object or creation of magical force, it is disintegrated without a saving throw.
Assuming the forcecage is fitted for your normal, medium sized druid body, you may find yourself squeezing in the confinement, but that shouldn't really affect you too negatively (outside the discomfort of having pressure on your many eyeballs). None of the medium sized beholderkin have a disintegration ray, generally to the relief of the adventurers who face them.
As this is a ninth level spell you'll only have one per day, but you are now a beholder and can turn your antimagic cone on the caster, preventing them from casting forcecage on yourself or anyone else. If you know you'll face multiple forcecage casters throughout a day, you'll either have to rely on your friends, or invest in ways to cast multiple 9th level spells per day, most notably either of the Epic Boons which let you do that.
†: Our options for shapechange forms here are fairly limited; the common beholder is the highest CR beholderkin that is not undead, and employed as what you're most likely to have seen. Since shapechange doesn't give you the Spellcasting trait you'll need one that has a relevant "disintegrate" attack, or either of the spells as Innate spellcasting or similar. For Innate spellcasting, there are none with disintegrate, but aeorian nullifiers have antimagic field 1/Day. The woe strider from Theros also has conic antimagic field, but is CR7 (and cones don't really work, see answer). The astral dreadnought also has a cone, but is CR21 so can't be shapechanged to (and you may have an issue of size). If there are any relevant monster forms I've missed, I'd love to hear about them.