Hi Alice
When you say your shoulders start to "freeze" i'm assuming you mean your dancing was too 'stiff' and so didn't look smooth enough.
It is often a difficult experience to go back to basics, but in many sports this is necessary to make an improvement. It's like making a step or two backward in order to make a lot of stpes forward in a new, more correct, direction. In skiing, it's used to cure 'intermediate plateau' as it's called.
You shouldn't have to re-work every single step - if you can develop the correct basic priciples of movement and swing, then this will work itself into all the steps you make and your dancing will improve in the long term.
You ask a question on bodyflight and controlling energy - -i can only answer as a man and Jonathan Atkinson (who runs this great site) will probably answer this much better than i can, but if you practise a simple natural turn in the slow waltz from a standing position..the very first movement is a softening of the left knee (for a man)with a slight lowering. This alone produces energy which is then transferred into movement by pushing from the standing leg (the left leg in this case) and the right foot lands on beat 1. During this movement the man starts rotating his body (clockwise) and then 'releases' his left side so it can swing through for the next step and the left foot lands on the toe on beat 2. The movement for beat 1 and the swinging action for beat 2 create the energy which allows the dancer to rise at the end of beat 2 and on beat 3. this energy must be controlled with sway (to the right for the man in this case) but in my experience, the sway comes naturally.
The details of this natural turn are complex and it's hard just to write them. Your coach will be able to demonstrate. But this swinging action with all the energy transfer is the key to expressive dancing.
As you improve and take longer steps, the energy involved increases, and if you can use it to your advantage, your dancing will improve.
don't tell anybody this, but when i'm warming up for a practice session i go round the floor several times doing slow waltz natural turns, and then reverse turns on my own. I find it puts me in the correct 'mood' as well as being a physical warm up.
Your other question about how does the man open his left side when going backward....he simply rotates his hips slightly in preparation for opening his left side. You should feel this movement, as he will lose some contact with you at the hips, and so you will know he is about to open in this way