In online learning, because deadlines and penalties for missing deadlines, are often assessed by a computerized systems, there is very little flexibility. You need to make a realistic assessment of precisely how long each assignment will take and schedule time in which to do it in your calendar. You need to treat that time quite as seriously as you treat class time. As most students tend to underestimate how long it will take them to finish assignments, you also need to schedule at least an hour or two a week per course of catch-up time.
If you have multiple online classes and are falling behind, decide which classes you need to complete as prerequisites or classes for your major, and prioritize assignments for those classes.
In general, online courses have much higher dropout and failure rates that regular courses.