Actually, managing your time is much more difficult unless you are using a time tracking software. Seeing the exact time you've spent on a particular task really helps you out to plan your day, week or month! You can see which tasks are taking most of your time and lead you to be unproductive. Analyze your work efficiency and results may be surprising when you enter your hours to the tracking tool. As soon as you've seen the excessive time you have allocated to a specific job, it's easier to drop and replace it with the prioritized one.
So I recommend you to use a time tracking tool such as BeforeSunset. It's really easy to use and gives you actionable insights for your future workflows.
That's a great question with complex answers. Right now I am just founder in a single startup, but for some year I've been leading two companies. My trick was: trusting and delegating things to do. I would specially recommend you to read this article: https://slack.com/intl/fr-fr/blo.... It helped me a lot.