When asking for more speed you don't want to lean forward. You really want to be pushing from your seat and hips. The horse has to power through the back end, up through her barrel and out the front. Because of the way you are leaning she can't get off her front end enough to engage her hind end, plus you seem to be nagging her with your feet and she is getting upset with you (evidenced by the tail swishing). Try and be more aware of her movement also, if she is bent to the left then she can't turn properly to the right and she will be stiff and mad about it.
If you sit deep, sit quietly active, moving with her instead of against her, then you will find she softens and relaxes more into what you are asking.
You might want to check out Horse Training with Chris Irwin | Horse Videos – Horse.com Video Library
there are some really good concepts on moving with your horse.