My experience of priest/druid/monk healers is that they all felt weak when first stepping into M+. All their heals seemed to hit for pittance vs tank hp.
The good news is they do scale well with gear and after a while you start jamming health into everyone with some left over. Resto shamans can be godly when geared up a bit and in the hands of an experienced player 
General advice is to be sure you’re using all your tools; Riptide should be on CD all the time, Healing Rain should be raining, and don’t be afraid to bust out Spirit Link.
However, if your group is regularly ‘on the verge of death’, it sounds like they are failing interrupts and/or standing in stuff they’re meant to move out of. There’s not much that that is unavoidable and should be taking players very low very fast.
If you’re pugging, consider running with a guild or community instead; people you get to know will generally result in a much smoother experience. If in doubt, consider joining Scared of Dungeons where we’re pretty good at getting people over the hump of a new class/role/anxiety and into the content they want to do 