I had good days followed by days of absolutely empty tank. I had to learn to measure out my efforts, still do actually. The worst bit was watching muscles atrophy.
Not much I can say that would be of comfort except that it can and does get better. Basically don’t measure yourself against others, just against yourself.
I still manage a couple of 24hr solos each year and a few 12 and 8 hrs – although no-one would accuse me of being fast 🙂