You could try just finding a slightly thinner bit, but still as a plain snaffle. If the bit is too big for her anyway, that is your main problem, as every time you use a rein aid the bit is just going to slide across her mouth before she feels it, making her feel 'dull' to the aid.
If you post a photo of the bit in her mouth, we can see just how much too big it is for her, and recommend how many sizes down you could try to go. Alternatively, use a wooden ruler, and if she'll let you, measure from the corner of her mouth on one side, to the corner of the mouth on the other side, going through her mouth. Measure in inches if you can ;) Take the measurement into the saddlery along with your normal bit, and they'll give you a hand to pick one that is a better size.