Have you tried her in a snaffle? I'd much rather you had her in a snaffle to start out. It's a kinder bit, and IMO a horse should go kindly in a snaffle before being moved into a harsher bit- flatwork/dressage of course, jumping is different I assume!
You don't want to aim directly at collection. People who do so usually end up pulling the head in and kicking, to get a really slow trot with a jamemd in head and yell out 'woohoo I've got collection!'.
Take things in slow steps, so first, you want her to react immediately off your aids. You put your leg on, she needs to move off it immediately. If she doesn't, she's not sensitive enough on the basics to even attempt looking at collection.
You need to have her soft and relaxed through all transitions and changes of rein.
Does she know any lateral work? Leg yield and shoulder in particuarly. These are great for lightening the shoulders-forehand and starting to loosen the back.
I also love counter canter to get the horse REALLY using it's back. Once you have a consistant, soft working canter, start riding her down the 3/4 line and leg yield her back to the track in canter. Gradually start riding shallow loops in canter and once she's comfortable with this, you can start counter canter on circle/turns etc.
MILLIONS of transitions every ride. Never ride more than 10 strides at the same tempo. So within a gait, ask for a little more length of stride, then a little less, a little more etc. etc. This keeps their brain ticking over, and will engage the hind legs and soften the back.
Collection isn't something that happens overnight. When I start with a horse it is at least a month of constant work until I even contemplate thinking of colelction itself. All the other work is just the lead up to it.
You need to have a horse that is consistant in walk trot and canter with a loose back, engaged hind end and is happy to bend, flex and move forwards, backwards and sideways with the horse happy to take a contact. From that, you can then start demanding the horse shift a little more weight onto its hind quarters and then you start thinking collection.
One thing to think about is that yes ultimately all your work is aiming at collection, but having a horses head down is not collection, and collection does not comeone day when you decide you want to 'learn' how to do it. Colletion also comes in degrees as the horse builds strength and confidence to transfer his weight.
The horses you see competing preliminary and novice are not in collection, they are performing 'working' gaits in which they are soft, relaxed and engaged- the step which leads to collection ;)
Sorry that I havent been thorough in my help here, it is near impossible to describe, explain and teach collection over the internet. You really do need a good dressage ho will not just tell you to use a gadget to pull the horses head in. We can generalise on the internet, but achieving certain degrees of education in a horse varies greatly between horses, they all react in different ways and you need to adjust your method in accordance to this. Hence why it is best to have 'eyes on the ground' who can help you in the moment.