I ASSUME you mean in dungeons, since you’re only 331? Then for one thing, your talents are off. I would suggest Power Word: Solace over Mindbender. It’s more consistent healing, and scales well with other talents. From there, you have the choice of two “builds”: an offensive build and a defensive build.
Offensive is Schism and Sins of the Many. This is an atonement centered build that focuses on doing a lot of damage (and atonement healing), but is very proactive and punishing if you slip.
The defensive build is Twist of Fate and Shadow Covenant. It lends itself more towards direct and reactive healing, and thus is usually more intuitive for new Discs. The defensive build has better tools for catching up if you fall behind. The offensive build helps you avoid falling behind, but if you do (common if you’re very reactive), you have less tools to catch back up.
Shadow Mend is an essential dungeon tool, especially if you are using the defensive build. It is your go-to if someone needs more than incidental healing (and the Depth of the Shadow trait on azerite gear helps even more). If more than 2 people need immediate and strong healing and don’t have atonement, skip individual shielding and use Power Word: Radiance. Ideally, you should have a few atonements up before big damage arrives. The tank should ALWAYS have atonement, and you can keep one or two others up on whoever’s taking incidental damage.
The key to Disc really is playing proactively. You should be mentally looking ahead to what may happen. You say you’ve played a tank, so you may already know which are dangerous trash packs that tend to go wrong or damage everyone. When you approach those as a healer, set up as soon as combat starts. Go in with 2 or 3 shields up already, set up Shadow Word: Pain on several enemies, and as soon as damage starts ramping, cover the whole group (either individually if there’s only 1 or 2 without, or with Radiance if there’s more) and go ham. A CD will also help a lot. Shadowfiend for a big boost in damage (and thus healing), or Rapture and put fat shields on everyone.
If you’ve got heavy tank healing needed simultaneously with incidental group damage, Radiance the group and Shadow Mend that tank. The tank and 2 others taking heavy damage? You can Pain Suppression the tank and Shadow Mend all of them, or Barrier and Shadow Mend, or Rapture and alternate shields between the tank and the others. EVERYONE’S taking high damage? Radiance, Shadowfiend, Shadow Mend anyone below 50%, then dps. Or Barrier - Radiance - dps, or Rapture - shield everyone - dps, or just Radiance - alternate Shadow Mend and dps. Toss in a Shadow Covenant if you’ve talented for it and everyone’s got atonement.
I certainly hope you’re using mouseovers of some kind, because they’re pretty much mandatory to pull this off. I’d also aim for Haste on both rings if you can. It helps a lot in that “feeling like I’m falling behind” feeling.