Review the original item:
Ursula clutched her head, trying to remember the correct formula to solve
the statistics problem,
the only thing she could focus on, however, was the stupid fight
she had in the car with her boyfriend Mac.
You chose the correct way to fix the error:
Ursula clutched her head, trying to remember the correct formula to solve
the statistics problem;
the only thing she could focus on, however, was the stupid fight
she had in the car with her boyfriend Mac.
Ursula clutched her head ... begins a complete sentence. The only thing she could focus on, however, was the stupid fight ... starts the second complete sentence.
You are always correct when you use a semicolon between the two main clauses that create the comma splice.