The most helpful post I saw when I was around the low 3* level was that "the biggest loss of stamina is from not being comfortable with the patterns yet."
If you're finding yourself losing stamina too quickly in a song, it's likely because you're punching a bit too far above your current reading ability, trying to compensate a lack of muscle memory with pure reflexes.
This can also result in wrist injuries.
Play some lower difficulty songs, and build up your muscle memory more until patterns start feeling smoother to play.
Avoid speed songs you can't A rank. The SR system is kinda garbage and can
severely under-rank really
fast songs when they only use simple single-note streams.