I struggle with that feeling. I've been bootstrapping my own business for years, and I have a strong inclination to feel guilty if I'm not "at work" while other people with more conventional jobs are at work. It can result in me spending a lot of unproductive time at my desk.
I recently took a year pretty much away from work, to do my house up (mostly building a home cinema, woo!). The business kept ticking over just fine with me doing 20 minutes of emails a day on average, and a bit more here and there to fix the odd pressing issue.
The time off reshaped my attitude to work-life balance quite considerably. After a year of not doing much work on my business, I no longer felt so guilty for not being at my desk. Now I go through phases of doing lots of work (I reckon that when the motivation is there, one should run with it), but I also try to do other things: meet friends for lunch, play tennis, learn Spanish, go on the odd little holiday. My instincts still tell me that I should be at my desk, working (or at least at my desk), but I've got better at seeing them as illogical and fighting them.
A huge perk of having your own small business is having the freedom to organize your time the way you want to. If you can have a successful business without having to put in lots of hours, that's great. But you have to give yourself permission to do this.
Sitting at a desk wasting time is pointless, but I can associate and I think it's a natural inclination. But it's a very unhealthy inclination, in my opinion much more so than the inclination to go and do something completely different, something fun.
This isn't to say that you shouldn't be able to find productive business-related things to do. Without knowing a lot more about what you do and what you're looking to do with your business I can't really comment on that. But with almost 15 hours a day at your computer, and "no life" (your words!) I do think you might do well to try and address your work/life balance, which may involve tackling the deep-seated feelings or instincts that have you sitting at your desk so long when you're not being productive.