It is possible to lose a healing. I remember one woman who had terrible vision. She was virtually blind. After prayer, her sight improved dramatically. But as she looked around, she thought, "I can see, but I am not supposed to see!" Her healing just went away. There was no way we could get it back. It just left.
This is what happened to Peter when he saw the Lord walking on water. Peter wanted to walk on the water as he saw Jesus doing. Jesus said, "Come on out." So Peter got on the side of the boat, put his feet on the water and started walking. But then Peter began to look around and became frightened. He thought, "I am not supposed to be walking on water!" The minute he started to think that, he started to sink. Today, there are people who receive a healing and then think, "I am not supposed to have this," and then they lose it. The devil has a way of coming to people and saying, "You are not really healed. You really still have that disease, and you had better go back and take your medicine and put your braces back on."
I know of one instance involving a man who was healed of multiple sclerosis. The doctors said he had a remission, but he knew it was a healing in answer to prayer. For a year, he had no evidence of the disease. Then one weekend, the entire force of mulitple sclerosis began to come upon him. All the symptoms started to come back, and for an entire weekend he wrestled against it, saying, "Satan I will not accept this." In the name of Jesus, he continually proclaimed that he was well. When that weekend of struggle was over, the symptoms left him. He has not been troubled since. But if he had given in to those symptoms, he would have once again had multiple sclerosis, and probably would have died from it.