Supports will be inevitably level gapped in a high kill-rate game. Don't feel too bad about it.
One thing to do is to make absolutely sure you stay alive during engagements, be as far toward the "back" as possible. Another way is to hang around the carries when they farm or clear jungle creeps to soak up that exp. Occasionally, if there's a lane that's empty and no one's farming it you can waltz in and throw down a few AoE spells and then get out before you become a target. Also applies to jungle creeps like the Medium Satyr camp, the Wolf camp, and any Easy camp. That's pretty much all you can do.
Stay alive. Mooch off of farming.