Everybody who ever had to take a long distance flight knows this sensation. As soon as the door is closed on the taxi taking you to the airport, all your preoccupations disappear and you fell instantaneously relaxed because there is nothing more that you can do. You spent the last hours frenetically trying to put everything in order, sorting important emails, preparing the baggages, calling the credit card company. After the door closed, the two only things you need to keep an eye on is your passport and your credit card - everything can be fixed as long as you still have these.
Well, resigning is just like this. You did your best to leave things in a good shape, but you no longer have access to the servers, or the code or the company's emails, so there is nothing more that you can do. Then you feel this deep serenity where your mind is freed of all preoccupations - you know it will not last, but you appreciate very minute of it.
And there is another parallel that I can draw between resigning and long distance flights: Because I woke up each morning between 2:00am and 5:00am for so many years, I now have to recover from work-induced jetlag.