This champion falls quite a bit in later stages of the game and is kinda cheesy as most midlaners never have faced against jarvan in lane so they might not realize how strong your burst is in first levels.
So against most matchups the gameplan is as follows levels 1-4 try get some decent q poke in to make your enemy be in lethal range of q+e+aa+ignite combo then just hit them with surprise combo out of nowhere for free 400 gold.
After very early game regardless if you were able to kill your enemy laner or not try to rush tiamat and boots and just start to shove in mid very hard with q+aa+tiamat combos and then roam as your champion was designed to be jungler your roams are extra strong in early-mid game. with this simple strategy and some experience of playing jarvan mid you should be able to snowball many games for free win.

So if Earlygame did not go your way or game was just prolonged for whatever reason and you reach early parts of the lategame and still are rocking that full on damage build it will become increasingly hard for you to find advances but at this point (+30 minutes) they do become even more valuable if you are able to find your way trough to enemy squishy that still dies to your combo at this point. So my plan for lategame is always just to try find some easy picks with clearing vision of enemies (red trinket+ control wards are your friends) but if enemies do not walk aground alone stupidly your best bet is just to group with your team and try to suicide for enemy squishy to start a fight.