A few thoughts in no particular order:
The 'Mechs are incredibly tough if you don't kill them the "right" way:
This isn't a first person shooter where you can get one and two shot kills with some careful precision. In order to defeat a Mech, you must either
Blow out his engine or
Destroy both of his legs. Each of these outcomes requires
roughly the same number of hit points of damage to accomplish, so take your pick.
Spreading damage over the entire Mech body requires more than ten times the damage of coring a center torso directly!
Focused fire is superior fire! Your opponents will not be shy to concentrate the damage of three or four Mechs on you. As an Atlas, in particular,
you're a very tall and easy to spot victim. With respect to the paragraph above, you need to
make it very difficult for them to keep hitting the same plate. Keep moving. Turn your legs (A / D), make your speed variable (W / S), and turn your torso away from their shots (look in another direction) so they're wasting ordinances on your side torsos, arms, legs, head, anything that isn't your precious reactor! Don't forget to also stay near friendlies. More targets for the "bad guys" means you might not get picked off first. If there's other heavys and assaults, don't be afraid to strafe behind them if you're taking too many hits.
Some weapons are more beginner friendly (and latency friendly, to boot!): Lasers are the every-man's weapon. They hit with pinpoint accuracy, have no travel time, and burn for about one second, giving you time to correct a poorly aimed shot. With practice, you can lead these on a laggy connection far more easily than AutoCannons. ACs are a much more difficult weapon to use. Their bullets travel fairly slow, are susceptible to gravity (will drag down over time), and have a firing delay which compounds lag.
Your reliance on the AC/20 to get kills for you might be your downfall. My gut feeling is that your tonnage would be better spent on lasers. The AC/20 requires careful precision due to its low ammo capacity, range and slow projectile speed.
I cooked up a recommended Atlas build for you. We need to get your confidence back, so I'm recommending a "training wheels" fit. It's competent at short and long range but wont win any gold medals. Here's a link:
http://postimage.org/image/a8tcrf8w5/. Please note, I used Artemis IV on the SRM6s.
Use the last empty ton for another ton of SRM ammo. Turn off "Auto Rearm" in the garage- artemis will suck your wallet dry. Use the free 75% ammo each match. Your goal each match is to completely deplete your missile stores without wasting any shots.
There's over 560 damage in the missiles alone.