how can i fix my gear box?

i have a 1999 subaru automatic. when i engage my forward gears, the car wont go. it can however reverse with ease. i have first change the gears to D2 and step on the gas really hard before it can move. and when it does move, it jerks up with a force. the tires make a noise like i applied too much gas. my hydraulic fluid seems to be okay. what could be wrong with my gear box?