Hi @KimberlyAustin,
Your school/institution is likely choosing to remove your old/completed Canvas courses or not show them to you. Each school/institution has the freedom to decide how to handle their enrollments/courses in Canvas so you'll find a lot of variance in this area. For example, the institution I work at keeps courses available to students for 7 years, but I know others remove access immediately. To find out if the behavior you're experiencing is intended, I'd recommend reaching out to someone at your school's Canvas team (you'd usually find them in an eLearning, IT, or Digital Education area/department).
I hope this info helps!
-Chris