please help (Medieval 2: Total War)

Of course, I understand, pure blood, a direct heir - a boy should be, but daughters can be married off to a husband and one of the husbands will be the heir. It's still a game.

The heir is serious, especially considering that he must be taught, and you can teach only after coming of age.
Yes, and do not forget that inheritance comes from the current ruler, i.e. the regent will not cede power and the inheritance will already go along his line.
So the choice is yours, either we cheat and develop, or let it go for good luck and find serious difficulties (desertion, extra losses in battles, etc.).