Try working front handspring with a quick heel drive several times before working the 1/4 turn. You might just have to go back and mentally reference the beginning of the front entry. I've also seen it help to put a panel mat (tall ways) in front of the vaulting table (it won't be too high) so you have to reach over it. The visual reference might help you stand up on the board and delay putting the first hand down too early, if that's part of the problem.
im not sure what you mean of turning to early but... make sure when you do the round off over the horse, well i guess its a table now, to make sure that your feet are going over your head and not around the side. its also ok to have your first arm bent and making sure your second arm is straight so you can bump.
It may be helpful to think of it more of a handspring 1/2 turn before hitting the horse rather than a roundoff. A roundoff is a hurdle followed by reaching down to the floor whereas a handspring is a hurdle reach up with the heel drive eventually flipping you over. You still need to be reaching up after hitting the board, then doing the turn.