Imagine being caught outside in a blizzard with no coat or shelter. It is freezing, the winds are howling, your toes and fingers are in severe pain, and all you can focus on is the misery that you are in. It feels endless, and like life is over. However, once you go back inside, your toes and fingers start to defrost; it's painful, but eventually feels good. Then, weeks later, as you think back on that storm, you get slight chill as you remember it. After the chill, you feel nothing--it is over now, and you are living a good life, so there is no need to fret over it. There is just a blank stupor when you try to recall the details of the cold. And eventually, you let go of the pain and move on. You can barely remember what it was like to have been freezing.
This is the same as going through great trials or pain in your life. When you are in the middle of suffering in any form, it hurts and feels like it will never end. It is painful, you hurt, the horizon isn't visible, and you can't imagine life outside of the pain and misery. However, you do get through the pain or trial, and when looking back on it, it is easy to forget and move on. If you think back to a hard time that you had in your life, then it might bring a tinge of discomfort; however, it's hard to feel the same again, and you let go of the memories, moving foward with your life.
Dickinson uses the freezing cold analogy to help us to understand that going through a bad time will eventually end, and we will forget about it and move on, just like when we are freezing and warm up again, the cold is forgotten. I hope those thoughts helped; good luck!