Confusing workflow

No due dates. Task disappears from lists when i schedule them.

How I want it to function:

I want to create tasks in lists. Each task has a due date when they must be completed. I will work on the task off and on up until the due date. What i want to be able to do is to schedule multiple work sessions for each task until it is completed. Hence, it should not disappear from my list just because i schedule it. Also, hard due dates is a must. I don’t get why so many developers miss this; it is equally important to know when i will WORK on a task, as it is to know when it must be DONE.