they realize taht it needs more time to regain their strengh and this rebellion ain't helping anyone.
The Empire chose to fight. They chose that over any attempt at peace. Don't fault Ulfric or the Stormcloaks for that. Ulfric forced their hand, forced them to make a move, but they chose civil war. Don't act like the war was something that happened to them.
Empire didn't choose civil war. Ulfric chose it when he decided to rebel. Yes there were a few number of ways they could have gone. But they chose the one which presented the smallest risk. Would war against thalmor be a better option? I doub't it.