Well if the conflict in this story is that Gurov and Anna cannot be together because they are both married and she leaves the seaside, the instability comes when Anna starts visiting Gurov in Moscow every two to three months. Both are exhausted and worn out from this situation because they want to be together. The reader doesn't know what they are going to do, causing instability. The final stability comes from the very final part of the story when Gurov tells Anna that they will be together, and that even though it may be rough, sometime in the not too far future, they will be together. That is the final stability. The reader can go away knowing an end, a stability to the story.