It is clear from your excellent description that you are suffering from a Pilonidal sinus. This is not a serious or dangerous condition, but can be very persistent and cause a lot of pain and unpleasantness.‘Pilonidal’ is derived from two Greek words, and means a ‘hair-nest’. It is called this because hairs penetrate the skin, become partly buried in it, and cause infection - hence the pain and pus that you have noted.
Careful examination - which you cannot do yourself because of the position - would reveal one or more holes (sinuses) in the skin, usually in the midline between the buttocks, often with a tuft of hairs protruding from them. There is redness and swelling, due to infection.
This condition, once it has persisted for a while, tends not to heal, and will require active treatment. This usually consists of a small operation, in which , under anaesthesia, the infected skin ‘tunnels’ are laid open by the surgeon. After this he usually leaves the wound wide open, with pack dressings to keep it so, the aim being to let the wound heal up from its base. If it heals by bridging across the surface there is a danger of the whole problem recurring. The operation may therefore be followed by six or more weeks of dressings while the wound heals, but the final result is usually very good.
There are operations that can be done, but this would be based on an assessment at the time by your surgeon. You should seek attention, and I hope you will soon be well again.