Imagine a stick or a pole that runs from your shoulder, to your but, to your heels. Try putting your shoulders back too and it will help you to extend your back and sit up
Shorten your reins but dont pull harder and make them tighter just have them shorter so you will sit up taller
I have the exact opposite problem! My friend leans back in the trot, and my trainer told her to practice 2 point at the trot. When she finished she was not leaning as far back anymore. Hope this helps
Make sure throughout your ride you are always saying to yourself " lean forward" or in every corner remind yourself to lean forward
Put a strap around your horses neck and hold onto it to get used to sitting forward
Close your hip angle. Some other thing to keep in mind: Put your weight in your heels, balance yourself, and use your upper calf to steady yourself and keep your leg on the horse. It's all about balance, and having a closed hip angle is important. Hope that helps!
When you lean back too much your abs disengage. So try thinking about tucking your pelvis underneath you and contract your ab muscles. This should help stabilize your seat and give you more balance as well. You can also try and imagine someone pulling a string attached to the top of your head upwards.