Must is a modal verb, so only a verb in infinitive form can follow it.
Interestingly ''must'' IS a past form... the present form is completely obselete.
To answer your question, to express ''must'' as a past obligation, use ''had to''. You'll find this in any English textbook.
''She told her husband that the doctor said that she had to go on a holiday...''
(I cleaned up the rest of your sentence as well.)