This is super common with green horses, I work with several including 2 of my own and we work on this very often. There are few things you can try to help with this. One being sit up and keep your shoulders back and elbows locked, talk to your mare and give slight half halts when approaching a fence. I know cross country you sit a little more forward and enjoy a forward pace but just take it a noch back to work on the distance. Also like others said Trot fences with Trot poles before and after are such great training methods to help with horses racing to take off. Also try an gymnastic line to help keep her jumping off her hind end. All of these exercises are great to help with striding and big spots! Hope you find this helpful!
It's very typical for a green horse to do this. Honestly, I'd let her make the mistake over and over until she learns. When she finally understands try setting up poles before the Jumps and make her wait and try trotting to them and really holding her for the distance you want
She could be trying to be figure her distances out, or she may not trust your judgement when it comes to take off time, so she jumps early. I rode a mare that didnt fully trust my judgement and she did the same thing in the hunter ring