Jungling got a lot harder in 2015 so now it's a tricky place to navigate early on. Things to keep in mind:
Armor Seals always. Make sure you're running combat-relevant quints as well, not things like Move Speed. Start Q to make sure you start with a maxed Blood Well. Make sure to start Krugs or Gromp to get a combat-relevant buff for your first clear. I'm pretty sure every champion in the game can jungle up until their first jungle upgrade with no issues.
Medium-mobility melee champions' biggest difficulty is gauging engagements. If you're not familiar with the types of fights you can take as Aatrox, you end up Q'ing in and then just dying. On top of that, junglers get a little bit less income than laners, so you'll be on average less farmed than your opponents. So I'm not surprised that's been your initial experience.
The final thing to point out is item build. What I see being most successful is one core damage item, either just your jungle enchant, or in addition to the enchant, and then building tanky. This should keep you pretty relevant in terms of damage output, while also allowing you to sit on the front lines pretty well.