In the question if tank healing at lower levels, if you’ve fully hot’d, barked, are spamming regrowths, and can’t keep the tank up, it’s likely in most cases the tank has over pulled and/or isn’t actively mitigating enough or any damage.
It’s helpful to have tools available to diagnose these issues, just about any meter will do in this case. I use details, it’s very easy to use and offers quick access from the meter itself to see what’s going on. In the case of tank damage mitigation, looking at the auras and scanning for uptime of key active mitigation and defensive cool-downs of the tank, should help you decipher wether or not they’ve over pulled or are just not using their kit properly.
At level 55, it’s likely you’re running into new and returning players that are learning/relearning the game, or you could just be literally healing a kid who is having a good time smashing buttons.
When you run into players that are for what ever reason making things more difficult than they need to be, be patient, be constructive, and accept that being a healer is often a thankless job. Besides, tough groups force you to push the capability and confidence in you’re healing kit, and is honestly good practice for doing difficult content later in the game.
When you do encounter a good tank, they’ll feel like they don’t need much healing. They will watch your mana, though it is also your responsibility to speak up and drink as often as possible. They will pull what the group can handle, position mobs/bosses to reduce group damage, and just generally be aware of the what the whole group is up to.
In the of the day, I’m not saying you should unreasonably stick through a dungeon with less than desirable folks. I’m more saying that as healer you’ll be inadvertently tested on a regular basis. Sometimes we’ll be faced with situations where the group really shouldn’t make it, and healing through those situations is personally rewarding and a wonderful reinforcement of your class/spec/personal capability.
Good luck out there healer fren:)