Front handsprings always come before round offs! If one does not have a front handspring, he can not be expected to do a correct round off. First, make sure you have VERY good front handsprings. Then, do 3 front handsprings (one at a time) and then 1 round off. Repeat over and over. Make sure you reach as long and far as you can before you turn in your round off. Pretend you are doing a front handspring and then at the last minute make it a round off. This will help you go straight. Kick your back leg HARD over your head in a STRAIGHT line.
I speak from experience, one of the most difficult things in gymnastics is to do a perfect round off. Honestly, a correct round off is harder to do than a correct double layout, double front, giant, tchatchev, you name it. How many elites do you see doing perfect round offs? The answer is almost NONE! Round offs are hard, but they are worth working on and making perfect. That perfection will spread to other areas of your gymnastics. Just make sure you are conditioning a LOT and you will be ok. Handstands, core, and leg lifts!