Be firm some horses at old or lazy and there's not much you can do
Sit up straight and deep in the saddle and use a firmer leg when you see her slowing down. You can also use spurs or a crop.
If I ever ride a push ride I like to squeeze ever other stride!
Use a lot of leg and keep leg on her and if you need to use a crop you might not need to use it because on some horses with just a crop being there it encourages them to go more.
Also, once she starts cantering, how can I keep her in a canter?