I would try reaching back further and also work on strengthening your arms and make sure they are also together. A drill I used to work on was popping my harms off the wall and then for your legs I would work on and keeping them together and snapping them do at the end. You want to to try and be in a handstand in the middle of a standing backhandspring before you snap down also really stay tight and use your core! I used to have the same problem! Hope that helps! Good luck! You got this!
Sometimes it can help to do it out of a handstand or cartwheel because of the momentum, I would start doing very momentous handstand back handsprings, then gradually as you start to get better use less momentum from your hand stands and start doing it with just a regular back handspring.
What really helped me when I was struggling with this is focusing as much as you can. Before you throw it picture a perfect back handspring in your mind. And, if you have access to a trampoline, practice it on there until you have perfect form.