Like the only punch I can throw going backwards is a checking hook, I have to stop or go forward to throw a jab cross or any other punch in an effective way.
Jabbing as you move back, assuming an orthodox fighter, is pretty simple. Your right foot moves back, and you jab as you push off (a bit) with your front foot. When you bring your left foot back, throw the right hand.
Keep your weight centered between both feet. Sounds like you're leaning too far back or leaning on your front foot.
