Why did Pharoah agree to allow the Jews to leave and then change his mind, after a number of the plagues?
It is true that Hashem hardened his heart and made him unreasonably obstinate, as the passuk says. But why did he agree and then reverse his decision? Why not have his obstinacy all the way through the plague and never agree at all to let them go, rather than saying yes and then changing his mind?
When the plague was in progress, when we are in distress over something, it might be too much to handle and we need to make the problem go away. Sometimes we will even agree to anything just to make the pain go away. Just to get rid of the problem.
But once the pain subsides, once the troubles are gone, the memory fades. In hindsight, it does not seem like it was really that bad.
Only after the troubles have past, when one is in a calm state of mind with relative tranquility can he then think back and say it was not such a big deal and then reverse his decision. During the time it happens, he just wants to get rid of the pain.