I've had cystic acne that lasted for about 3 months. That is as bad as it gets, I think.
In those situations, the doctor usually injects a shot into the pimple to totally flatten it within 48 hours.
But in your case, somthing that has been there since December, thats over 6 months - I doubt it is a single pimple.
It might be a recurring cystic that never ends. Check w/ the derm and if it is a zit - a shot will make it go away in 48 hours.
By the way - does it hurt? If not, then there is no deep skin inflamation - therefore - not a CYST.