You are absolutely correct. I’ve never met anyone else with my level of concentration. So other than what I experience first hand, I’ve never seen anyone suffer from fatigue. But, while others can seem jealous about my small accomplishments, I actually get jealous that they have lives.
Is it about your side-project? You really should only be doing a side-project for your enjoyment and the challenge. However, if you stress about your productivity you might lose sight of the reason you started a side-project. I am the person who can concentrate 16 hours a day, or even days on end without sleep or breaks (in fact, i'm well known for it). It doesn't mean my projects get done faster, the real result has been making those around, like my family, feel neglected. I say enjoy your time with your girlfriend, focus your productivity at work where it matters, and just enjoy your projects. --- There is probably a good argument about the mental health of those, like myself, who can concentrate for days on end, months at a time. Actually, I think its great that you can relax after 10 hours a week on your project. I can never shut my mind down long enough stop working on my projects, or even sleep.