Fretting hand technique
Fretting hand technique on the classical guitar can be physically arduous due to the number of voices we have to play simultaneously: often we require held chords underneath a melody line. Or a bass line, middle accompaniment AND melody all together. There is a lot of physical fretting going on and it can cause aching and tiredness in the hand or arm. To build up stamina make sure you use sensible and efficient fingering, learn pieces slowly in small sections, with relaxed accuracy. Stop and rest for a moment if aching occurs. And practise regularly!