The past tense of "must" is "must." So "she must go" in the simple past is "she must go." As Thoreau wrote in "Life and Letters To-Day," (page 227), "If Thoreau went because he would, Hawthorne went because he must." Most English speakers would work around this awkward situation by saying "she HAD TO go."

The past form of "must" is "had to". She must go now --> She had to go yesterday.

Yes, "must" is actually a past form (the present form doesn't exist anymore). Older writers use must in past tense. The modern solution is to use "had to" when moving "must" to the past. The meaning is clear. "She had to go."

She must go = she had to go