Review the original item:
Tajuana is a very conscientious student when
she makes a B, she is so upset that she lies awake at night, staring
at the ceiling and worrying about the effect this "bad" grade
will have on her GPA.
You wanted to fix it this way:
Tajuana is a very conscientious student,
therefore when she makes a B, she is so upset that she lies
awake at night, staring at the ceiling and worrying about the effect this "bad"
grade will have on her GPA.
To add a comma and the conjunctive adverb therefore between student and when would cause an equally bad problem, a comma splice. A comma splice occurs when you have two complete sentences joined with a comma alone. Tajuana is a very conscientious student is the first complete sentence. When she makes a B, she is so upset ... begins the second complete sentence.
The spot between student and when needs a stronger break than a comma and a conjunctive adverb.
You might want to consult the rules for fixing comma splices and fused sentences.