There is a BIG difference between a horse stretching out on a looped/loose rein, and actually working long and low. 90% of the time you see a horse on the forehand and the rider thinking they have achieved long and low.
True long and low is very difficult, for both horse and rider. It requires a good degree of feel and timing by the rider, and a great deal of balance, strength and confidence from the horse. The back needs to stay round and swinging, with the hind legs coming through and remaining into the bridle. This position is hugely difficult for any horse to maintain, and as such it requires a great deal of physical conditioning before they can hold it for more than a few strides, without falling on the forehand.
First - get rid of the inside side rein. If you want to use one side rein, put it on the outside. By blocking the inside and opening the outside, you encourage over-longitudinal-bend through the neck to the inside, allowing the inside shoulder to take all of the weight and the energy generated behind simply falls out of the outside shoulder. Its an effective method of intentionally putting a horse on the forehand.
You need to establish a good contact on a shorter rein before even thinking about long and low. Many people work backwards - trying to get long and low first, with the assumption that it will make the horse loose. Yes it will often relax a horse if they are dumped on the forehand and allowed to put their nose on the ground, BUT, it is useless for building topline and conditioning.
Get spot on basic transitions on a shorter rein, with the horse willingly working up into the bridle, with active, carrying hind legs. You can then gradually give the rein, allowing the horse to take the contact forward and down. Initially allow only a little, then bring back up into the shorter contact. Every time you feel that the horse is in the contact well enough, test that contact by giving and seeing if the horse follows. If he does not follow, then you are not doing your job properly.
It should feel as if he is gently chewing the rein from your hand, with NOTHING changing in his body except for the slightly longer/lower neck, at this point. If you allow too much rein and he dumps on the forehand thus losing his balance, then you will have to hope you have a forgiving horse!
It is up to us as riders to establish and maintain a horse's balance under saddle. If we skimp on this part of the deal, the horse loses confidence in our abilities. Then later down the track when we start asking for even more demanding movements such as the half pass or pirouette, is the horse going to trust our judgement and ability to keep him in balance?