As for matchups, Leona is one the easiest ones.
Her main counters are Thresh and Janna since they both can deny her Zenith Blade engage with Flay and Howling Gale. Maybe your ping is pretty high and you dont have time to react but, as soon as you see the animation of Zenith Blade smash your Q twice. Use Eye of the Storm on the one who she engages on, you or your ADC. At lvl 6, your Monsoon will also stop her E.

Remember that Janna is not a win lane support-type. She just stays there and helps her ADC through the lane phase. Her moment is mid to late game, when you will peel everything and shield your team.

Ardent Censer passive is given by our Eye of the Storm and Monsoon. Locket of the Iron Solari grants it too. Also Redemption. Abuse that OP item and climb high in the ladder.
The downpoint of Janna is that you relay 100% on having a good team. By yourself, you can do nothing.

I suggest you playing Karma(more aggresive style but with huge shields), Nami(more passive with CC, heals and more peel) or Lulu (mix between peel/shields/damage) as they are better champions on their own to carry a lane or even a game. Also the runes/masteries/builds are pretty much the same as Janna's, so you can use same the same setup and playstyle. You will climb faster as you get to master those three champions.

Lane phase now counts more than before, you need to win it, not just go even, so Janna is a bit worse. You cant relay on your jungler ganking for you, since you offer so little engage/CC.