Must establish true forward momentum. I always start in a 20 meter circle and push him forward with my seat and legs until his back is up and head is down. Then I slowly start using my reins and asking for a bend.
I found out from having to start from square one with my horse that it really helps to work on basics. Straightness and transitions especially! My horse likes to lean on my right leg and making a conscious effort to keep him straight and in between my legs has made a world of different. I’ve noticed this also works on other horses. Make sure they are not on their forehand when going around, you want to feel them useing their hind end to push them forward. Some horses will just soften from that, no fiddling with the reins needed, but if their head is still up and their back is still hollow, hold your outside rein firmly, and touch and soften with your inside. You should feel them start to lift their back and move into the contact.
Try a circle and leg yield them to the rail when you cut across the part of your circle that’s across the arena. Also doing figure 8 and serpentines to change the bend is really good for them. Bend their ribcage
What helped my horse a lot was riding a circle in the middle of the arena and made this smaller and bigger and then try to get your horse a little bit more collected when you ride the circle smaller. Try to do this with your seat and have a soft hand. When you ride the circle bigger again push the horse forward with the leg. Try to do as less as possible with your hand and bend the horse around your leg and keep the horse relaxed. Then you can start to ride transitions between trot and walk. Make sure you give the horse the rein when he needs it let him stretch a lot in all gaits. Another thing you can try is shoulder in on the circle, just about 3-4 meters and then ride forward out of the shoulder in and give the horse a little bit of the rein. I hope this can help you.
Hold light contact on your outside rein, while giving and taking softly on your inside rein. This will help them get into a frame as well as softening up. If your horses head goes to low, pick your hands straight up.
Legs before hands every time.
I sometimes find if horse is evading that giving something to think about helps. Lots of change of direction, circles, serpentine etc... get them focused... it's often easier in trot initially as well