If your day list is always bigger than your day, start estimating how long tasks take, aim for a realistic total, then trim. Show only a few tasks at a time and watch the clock so you plan a day you can actually finish.
What can help
Duration Estimates
Add how long each task will take, so you can see when you've planned more than fits in your day.
Read more about Duration Estimates
Truncated List
Show only a few tasks at a time (try the 1-3-5 idea, or just three). A short visible day keeps you from quietly stuffing it past what actually fits.
Read more about Truncated List
Day Progress Bar
A bar across the top of your day shows how much time is left and how far through your tasks you are, so the day doesn't slip away unnoticed.
Read more about Day Progress Bar
Ideas to try
Add durations as you plan. Type a duration in the title, like Write report ~45m. Marvin adds up the total above your list, so you can see when you've planned more than fits in your day.
Plan, then trim. If your total is bigger than the time you have, move a few tasks to another day before you start, not at the end of the day.
Try 1-3-5. Plan one big task, three medium ones and five small ones. Use Truncated List to show only a few at a time so the rest doesn't distract you.
Watch the day. Turn on the Day Progress Bar. With your work hours set, it shows how far through your day you are next to how much of your list is done. Turn on Red bar if behind if you'd like a clear signal.
How to turn on a feature
Open the main menu and click Features, or press S.
Search for the feature by name and turn it on.
Choose where you want to reach it in the Access dialog that opens.
You don't have to try everything at once. Pick the one that sounds most like you, use it for a few days, and see how it feels.