Hi Black Beauty 94,
Balance and muscle is not something that develops overnight. It is a long term process that is forever changing.
It sounds like your horse is struggling with the basics at the moment. Balance and muscle building comes with a horse who will go forward and use its hind legs. So right now, I would be focussing soley on improving your horse's reactions to your driving aids. He need to willingly go forward, with active hind legs.
When you can achieve this, you will find that his balance and muscle build up will drastically improve.
Fo now, get the notion of collection completely out of your head. This is something that takes a long time of skilled riding and training to achieve. Right now your horse is not ready for collection. You need to start from the bottom and work your way up - if the foundations of the Great Pryamids were not stable, they would crumble as they grew taller. Same goes for training a horse - that is why in dressage, we have the Dressage Training Scale, or the 'Pyramid of Training'. If you don't develop a solid foundation and rush straight to the 'fun stuff', your training will crumble.