Found out a cool thing before, not really sure when this happens but it wasn't available earlier, it used to be only two - normal and hard(+2 levels or so). I guess when you hit 20 the rest of the difficulties open up for the normal story missions:
Was a little concerned I has spoiled myself levelling to 20 before completing the story, and I'd have to just rampage through a load of "level 12 story" with my one-hit-kill everythings - but no! They provide some serious challenge with the "heroic" modifier enabled, I died a lot of times on that mission where you deploy the ghost and then fight some waves of enemies while he works
Also unlocked some daily and weekly super-hard missions - same story mission as before, but with more modifiers on. The one I did was Old Russia's "The Warmind" Along with "Heroic", it had "enemy melee attacks are far more damaging than usual". Was intense!
So yeah don't worry about over-levelling yourself