Without seeing it, its hard to say what you really need to work on... What it sounds like, though, since you need help getting your feet on the beam too, you're not square on your take off. Of you try doing a flip flop with your legs or hips out of line, you won't be able to stand up because your body thinks you're wobbly and won't stay up for very long.
Working on your take off and making sure you're in line may help. Make sure you're kicking that lead foot towards the ceiling straight above you, and not just whipping the handspring.
You also may want to practice handstand-stand ups, making sure your hands lead the stand up and not your shoulders (I.e., crown as soon as your hands leave the floor and pull them up to stand).
That's about all I got.. Good luck!