Dont lose nerves. There is solution.

Immediately retreat from europe and steppes. Concentrate on holding north africa and Middle East. You may establish you borders in balkan lands, or even constantinople.

The reappearing factions will spread their armies, and they will start fight with each other sooner or later.

While you retreat, leave only burned ground. Demolish every building, if you abandon the province. Money is allways usefull, and we dont want to power up AI with advanced troops. Also, abandoned provinces will be rebelious, so even if AI takes them, there is very likely, that they will be having troubles with rebels also. And, if you empire will be smaller, you have better chance to seal alliances.

You situation is hard for one reason. You conquesting plan is not very good.

If you want to conquer any province, look at the religion of inhabitants of this province.
Your religion(muslim) will cause problems to orthodox, catholic and pagans. If this province you want to keep, lower the taxes for couple of years, build mosque and send some alims to convert people to your religion. Build towers and border forts also.If it is shore province, strong fleet must be keept on sea. That raise loyality in the province considerably.
And when loyalty will be far above 100%, and religion in this province will be muslim mainly, that province is yours, and will be less rebelious in future.

Keep the loyalty of the best generals, giving them many titles. I usualy send unloyal generals(with some mercenaries that i not need anymore) at the enemy. They will die, preferably with some enemy troops also

That slows your progres in conquering whole world considerably, but less rebelions, less problems.