(As Heavy) Cry sum moar leetle leetle man!
Jokes aside, Crysis 3's has a bit of a learning curve when it comes to managing your nanosuit energy (though IMHO Crysis 2's energy manegement was harder- you had 1 energy bar for everything, including running and power jumping -_-).
For any FPS I found that the key is to be conservative, anyone will tell you rushing does little good unless you have a solid idea where the enemy is or are prepared to take on at least 2 people comfortably.
Also, they've nerfed meleeing in Crysis 3, cos in Crysis 2 time between melees was too short, leading to exploitation.