Hi popsec
Check out:
CRC - Home This is Sylvia Loch's website and is very helpful. I can also wholeheartedly recommend her book Invisable Riding, really helpful for not just the dressage rider!
Sylvia Loch is a classical dressage rider and her principals are also those that the Spanish Riding School is based on.
It would be difficult to comment on what is happening when you are trying sitting trot now without seeing you in action so to speak. A common reason for the trot feeling very bouncy is tension. Do you have access to a good instructor who could help you?
Let me know what you think of the website and the book as well!
Steph