In my experience, when people have issues with standing tumbling, the issue is not with the end trick; the issue is the handsprings leading into it. Without powerful handsprings, in proper form, that speed up, riding a layout up will be next to impossible. If you are not blocking though your shoulders properly, and/or punching with your feet in the correct position, your body's natural reaction is going to be to rush the skill.
For example: if you are landing your 2nd BHS connected to your full with your feet in front of you, you are going to naturally travel back not up. Also, if your chest is down out of that handspring, your naturally going to try to arch to in order to flip.
Work on perfecting the technique leading up to the layout/full. Your back handsprings should be LONG, your body should be stretched back into the second one, and you should block with your feet behind you while lifting through your core as well as engaging your shoulders into your layout/full.
Hope this helps.
Coach Matt