Take a look at the original sentence again:
Carla has difficulties getting to her 8 a.m. English class on
time, she always watches too many late night
movies and then hits the snooze button on her alarm once too often.
You chose to fix it the correct way:
Carla has difficulties getting to her 8 a.m. English class on
time, for she always watches too many late
night movies and then hits the snooze button on her alarm once too often.
Using a comma and a coordinating conjunction like for is an excellent method for fixing a comma splice. The challenge is using a coordinating conjunction that logically connects the two main clauses. In the example above, for does the job because it means the same thing as because.