Runes/Masteries make a world of difference.
Picking up the mechanics of the game takes some time, definitely hurts if you can't play regularly. You still have 12 levels to go before 30, you will learn a heck of a lot more and maybe won't be Bronze tier by then
Jungler will usually be slightly behind in levels, should at least be the same level as your support & adc. The Solo lanes gain xp faster.
You can increase your leveling speed as jungler by grabbing the blue/red buff xp nearly everytime. I don't know if you're donating away the blue buff or not at your level, if not, then thats fine, just keep grabbing blue & red buffs, if you are donating the buffs away to your laners then what I normally do is get the creep low health then hold the 'S' key to stop all movement & attacks, letting the creep hit me while my laner finishes it off, I'll still get a good chunk of xp for the kill. If you move too far away from it then your laner gets the buff + all xp.
Also as a jungler you can't be timid, to really accelerate your leveling you should try counterjungling. Time enemy buffs and steal them before they get to them. Red+Blue respawn in 5 mins, dragon in 6 mins, baron in 7 mins. If Blue buff dies @ ~2:10 (it spawns at 1:55, figure ~15seconds to kill it max) it will be back up at 7:10. I normally ward enemy blue around 6:30ish if I plan to steal it. Red buff is normally taken 2nd and might be up around 8:30ish. 2nd respawns of buffs are in the 12-13min range. Keeping notes on respawn times is helpful. As a jungler you need to have a "killer" mentality. Avoid just passively farming in your jungle.
RE: Nasus, that's the thing with Nasus, he is a late-game threat after Q is farmed up. He works as top laner by just passively farming but can get harassed & denied Q hits, as a jungler Nasus can safely power up his Q and he isn't required to do tons of damage for his team during early game (junglers are typically tanks with mediocre damage). Nasus has great ganks though due to wither.
