Very few people know how to use sidereins properly. They put them on and the horse looks collected, but really they're tense in the back and neck and hollow as well, and most are behind the bit.
You're much better off just learning to be in balance together when you get a dressage trainer.
And do realize that there isn't just one dressage trainer around. Go on craigslist or anywhere and I'm sure you can find a decent dressage trainer to do a few lessons with.
Also, your horse doesn't need collection. She needs balance.
If you really want to help the horse, start free lungeing her. You can do it even in an arena. It just takes more effort. You ought to get the horse to stay pretty close to you and walk/trot/canter on demand in a calm manner.