dont tense up & keep your hips open. tilt your pelvis under SLIGHTLY
Think about keeping your heels down and your legs under you so you stay upright, if your leg is behind you you will tend to fall foward.
Trotting without stirrups. It will open your hips and make u sit on your seat bones. This exercise will make you work on your core muscles in order to keep the balance. To help yourself sit up and let your body just go with the horse's movement and let your shoulders go slightly behind your hips but only a little bit
If you're struggling ask someone to take you on a lunge.
This will not only help you with your muscles but also help place your leg in the right position and help you become more balanced, therefore, you will be able to keep your leg on and keep in the right position in the saddle and ask for canter at the same time
Maybe do planks and calf raises to build up your muscle and just really focus on using your core and legs at the same time, not just in your transitions.