I find that working too many hours outside of 9 to 5 quickly becomes lethal too. And working on side projects as a cofounder feels very much like cheating on a

I find that working too many hours outside of 9 to 5 quickly becomes lethal too. And working on side projects as a cofounder feels very much like cheating on a spouse for all the reasons you mentioned. One approach is to look for future needs and do quick startup related R&D projects in place of true side projects. This doesn't have all the benefits of real side projects though. And it has some real disadvantages too. It still feels like work and can lead to resentment and burnout if you aren't careful. But this the best compromise I've found.