Another thing is move her body around A LOT. Stop, back, rollback everytime she speeds up.
Sidepass, halfpass, reverse ark, turns on the haunches/forehand.
Basically get her really loose, and her body all in the right position. In order to lope slow the horse has be rounded up and engaged.
A lot of horses can't go really slow but you can definitely get it more relaxed and slower than it is.
You have to be patient though, you can go from top speed to lope. You need to take it in steps, work on getting her a touch slow then when you get that consistant work on getting slower again.
Repetition is key and over time it will come, but it won't come over night.