Riding around with your reins flapping in the wind and plodding around on the forehand weaving around the arena does not qualify as "long and low".
Riding a correct long and low, beyond a stretch for the neck and a pat as a break, is very difficult and should rarely be the composition of an entire ride. You have now taught the mare to plod along doing nothing, with no drive from behind and when you go to pick her up, of course she will evade it!! She's spend the last month doing extremely little in the realm of "work".
You should always have a contact on the reins, if she is not warmed up enough to stretch to the degree you want then your reins do actually need to be short enough that you have at least 2 lbs of contact in each of them. As she warms up, you can let them longer but you always must be holding a contact. Then ride in walk trot and canter, and transitions between, forward and she needs to drive from the haunches. Your half halts serve to balance her onto her haunches while remaining forward and you use your fingers and wrists to maintain a supple contact while she is driving up from behind and making the contact stronger. Then you have to ride to your line, don't allow her to go where she wants, she needs to be exactly between your aids and not drifting through either leg on the turns. Again, maintaining forward.
By you driving her forward, holding a contact, half halting and suppling that contact while keeping her between all your aids, then and only then will she be able to create a "bridge" over her back and being to carry herself in self carriage. When you go to pick up the reins from this correct long and low, absolutely nothing changes in your aids, only that your reins are shorter and you may need to drive more forward. Long and low also needs to be done in trot AND canter with transitions, transitions, transitions and more transitions to really help the horse to "unlock" and swing and come over the back.
They say money doesn't buy happiness -- well happiness doesn't buy horses!