You need to back up in your lunge training and get the responses you need at the walk first. To slow her under saddle you need a straight fence, paddock rail, barn wall, etc. Rider her about 6' off the rail and the moment she picks up speed turn her into the rail. She will stop and that's ok, just finish the turn and move away from the rail again. Repeat working on the rail like this until she figures out that speeding up means tight turns (harder work). She'll get so she does nice tight turns without stopping as she gets her hind quarters more engaged (collected). As soon as she will go 20 strides without speeding up, put her away as that is a huge reward. Repeat this exercise daily for 3 days if you can. Hopefully by the third day she is holding her speed. If she does keep this session to only 10 min then put her away. You'd be surprised at how well such a short lesson can be retained especially with the reward of no more work.