I'm guessing from your level that you're new to the game.
Nasus and Jarvan are fighters (Jarvan has a pretty flexible build path, so he can be a tank or an assassin as well). Nasus is specifically a Juggernaut, and Jarvan is a Diver.
Juggernauts are weak to kiting. This means that you/your team should apply slows on him so that he can't walk towards you as quickly. He can only do damage when he's close. Other Juggernauts include Garen, Darius, Udyr, Illaoi, Volibear, etc., and they all have the same weakness. Each has a different way to play against them, but kiting is the common weakness of the Juggernaut class. As a support, use your CC abilities on Juggernauts. Most supports have some CC in their kits.
Divers should be focused down. I'm guessing that in your game, Jarvan bought items like
. That means he should stack HP in order to do damage, and build little to no armor and magic resist. In that case, %HP damage should be used on them. Most tanks and many ADCs have %HP damage, and junglers that build
should try to focus them.
Overall, if you want a support that can protect the carries well, Wardens (defensive tanks) and Enchanters (squishy casters that focus on buffing allies) are ideal. Some examples of Wardens are
, and some examples of Enchanters are
. Janna is one of the best (if not the best) support at peeling, so you may want to pick her up if you want to keep playing supports.
On a more general note, supports can engage or do damage too, instead of just peeling. You may have more success as someone like
if you think the defensive playstyle isn't for you.