The stage of purity go endlessly and so-called it "Nirvana" comes very end of it. Nirvana is literally "death", perfect death is what Nirvana is and we accomplish it before this body reach to end - before still functioning remain we reach to that end of purity. That's what we are doing.
You may say you are going to sesshin and sit. That's a big lie. If you say to your friend, partner, "I'm going to die", no-one can let you go to sesshin so you say, "I'm going to sit." No wonder it is difficult thing.