I think your main problem is starting out with some very difficult battlefield roles (offensive sniping, cloaked assassin) without first grasping the main jet-infantry role.
The sensor jammer is good for stealth if you're dealing with a lot of radar-based defenses, like turrets. It also prevents you from getting an enemy tag when an enemy pulse sensor has you in range. Check the circle around your compass bearing. If it is green, you're in friendly sensor. Red, you're in enemy sensor. Yellow, you're in jamming, no color, you're in radar dead-space or out of range. Not a lot of people use it, although it can effectively keep you off the enemy sensor net if you avoid visual detection (which always throws a triangle over your head). It is also nice on a bomber or APC, since when active it throws a jamming bubble around everyone near you, keeping the missiles away unless visually detected.
Cloaking is still quite possible on the ground, the secret is to constantly jump, or possibly ski. Hitting someone blind while cloaked at full tilt with a shocklance in the back is tremendously satisfying. However cloaking seriously reduces your ability to burn your jets, so I recommend waiting until you master basic light armor tactics before trying it.
Basic light armor for a newbie (and most pros) is as follows.
Chaingun: engaging airborne or same-plane targets.
Spinfusor: engaging grounded or soon-to-be targets.
Grenade Launcher: engaging clusters of ground targets.
Energy Pack: defending yourself with maneuver
Flare Grenade: defending yourself from missiles
Mine: dropping on enemy inventory stations just to be a jackass
Until you get reasonably capable in Peltast armor with an energy pack and understand skiing, proper use of the spinfusor and game mechanics, trying anything else is going to be an uphill learning curve. If you find the super-athletic style of outdoor fighting impossible to grasp, switch to medium or heavy armor and defend the generator with a plasma gun and shield or repair pack. Or be a turret farmer. Or stand on the flag in heavy armor with an ELF gun just to be a tremendous bastard. Or see how many enemy lights forgot to bring flares with the missile launcher.
The options, they are so many. Tribes is like Team Fortress 2, if you could say you wanted the Demoman's grenade launcher and the Sniper's SMG with the Soldier's rocket launcher and the Medic's healing.
And lastly, AA turrets are nice, but I typically prefer ELF turrets for flag defense. Most players are too freaking fast for AA, but an ELF beam -will- hit anyone in range.