I watched and judged, or gave feedback on, over 60 dressage tests this month at Interdressage.com and one repeated piece of advice is to try to place the balance into the outside shoulder.
Horses are never symmetrical and on one rein the inside shoulder may be heavier (take more weight and fall in on that shoulder) than on the other rein; this is not abnormal and is one of the reasons we work at our dressage! To build up a balance between both shoulders, both hinds (one pushing one more able to flex and take the weight), from front to back and diagonally is the whole essence/raison d’etre of dressage.
Often the rider compounds the problem because they are told to bend to the inside. On the rein that the weight is already heavily placed to the inside shoulder the act of bending can put even more weight onto the encumbered shoulder and drop the horse completely on the forehand as well as the inside shoulder.
A very simple solution is to focus on your shoulder balance in training- first in walk and later in all paces. The video below shows how simply you can readjust the balance by lightly using the outside rein. In this video I am using nothing aside from the reins and evidently only one rein at a time. Of course Moralejo is relatively confirmed in his balance- he is croup high and so easy to put onto his forehand but the act of putting him into the outside rein transfers weight into the outside shoulder so that I can then return to true bend with a little inside rein flexion. As soon as the balance is lost and weight returns to the inside shoulder then it is simple to return to the outside rein feel to rebalance again. In time you sense the loss almost before it occurs and adjust without even realising it- the gentle art of rein use can be developed from these small beginnings.
Work those shoulders in balance!