Try with double reins but still with one bit. It helps with my 5 year old mare.
Hacking really helps, lots of uphill gets them engaged. When your going up hill encourage her to stretch forward and down. Keep your leg on and really make her use back and hind end to push her self up the hill. Make her go forward into your contact. You can build this up to trotting up hills etc.
Lunging is great too. I'm not a great fan of side reins as more often than not they are used incorrectly and force the horse into a false frame.
I recommend a chambon! This is a non-restricting training aid that using pressure at the poll to encourage the horse to stretch down, just make sure your horse is going forward and not on the forehand.
In the arena, transitions transitions transitions! Really get her listening. She needs to be in front of your leg. Focus on your rhythm.
Circles and lateral work are good too.
When doing flatwork, remember inside leg to outside hand. Use your inside aids to bend and soften her, all the while keeping a steady outside hand. Don't see saw!
Don't be too restrictive with the outside hand you have to be quick to follow the movement and not hinder her dropping her head.
Sorry for the long answer:) Hope I was of some help!
Try lunging in side reins. Start at the walk only and do like 5 min each side take off the side reins and then lunge like normal. Build up the time they are on the side reins before moving up a gait. Itll help put them in form and build the muscle so when you ask for it while riding they have the muscle.
I use a standing martingale (a very tight standing martingale) to start with. By putting your leg back so your heel is directly underneath your hip it gives you a better feeling of your horses side and so you don't ride so defensively. Work your horse is circles in a corner of the arena (or whatever space you use for riding). Something most people don't bother to look at is how light their seat is. When you thump down on their back it's a natural cue for them to move forward quicker. So by slowing your post and only brushing against your saddle at the post it'll help your mare to relax. This is something I've been working on with my very ornery mare myself and she actually picks up her head set and is more supple