Your endurance is probably an issue, but that will get better. In the meantime I'd prioritize end reduction and accuracy over damage when slotting your attacks and toggles until you have more slots. Smite is one of your best attacks, that should be in your rotation at every level. Shadow Maul can be difficult to use and it has a long activation, I suggest picking up Combat Jumping to help with positioning to maximize it's effectiveness. Plus Combat Jumping is the best thing ever. Overall Dark Melee has poor AoE, that won't be changing.