I think this is a combination of lack of tanking tools and bad balance and toning of Shadow Priest in low level content.
If you have been following the complains on the Priest forum, a major one is that the Shadow Priest redesign was not completed when BfA was released. For an incomplete spec at max level, I won’t be surprised if it is not balanced at low levels.
As others have mentioned, you are probably having a very uncommon experience as a shadow priest solely doing dungeons at low level for so many dungeon runs. Every leveling advice given today is either “play disc” or “heal dungeons “. (Or in other words, don’t play shadow.)
I think you may have come cross some valuable “beta testing” issues with low level shadow priests that should have been brought up to the developers.