Only do things that move you closer to shipping. I'd also look at ruthlessly cutting scope so you can get your product out the door. Most importantly, focus on making consistent progress every day or week so you can actually ship your product and start getting revenue from it.
you need to be very disciplined about quitting responsibilities, saying no, clearing your plate. A startup is more than a full-time job, do everything in your power to narrow your focus and avoid burdening yourself with commitments. in the same vein, be proactive not reactive. set aside a portion of your day for "work" and don't respond to emails or calls during that time. do what you decided to do, not whatever feels right in the moment. the feeling of progress will give you fuel to keep going.
I suspect the problem isn't so much the lack of hours in the day, but the ability to "see" your work and know what you're capable of handling so that you can actively make decisions about what to take on and what to defer.
In agile/lean circles we call this "limiting WIP" or work-in-progress. As human beings we have a finite WIP capacity, and we benefit from keeping it low so that we complete what we start (which has a powerful psychological effect). One way of doing this is to set up a personal kanban board so that you can visualize your work and control how much gets into your WIP queue, worked on and then completed.
A fantastic, easy-to-read resource for this is Jim Benson and Tonianne DeMaria Barry's book, Personal Kanban: Mapping Work, Navigating Life. It quickly explains the theory behind the technique (adapted from lean manufacturing techniques) and how it can be readily applied in a variety of contexts.
Nearly all customers I've coached to adopt this technique have benefited from it as it leads to a deeper understanding of the work they do and how they do it.