Try making an boolean instance variable on your Player object (Sprite) called Dashing, set it to "false".
The logic is something like this:
1) If Player is on the floor and Dashing is false, set (normal acceleration, velocity etc atributes)
2) If Player is on the floor and pressing Dash Button, set Dashing to true.
3) If Dashing is true, set (dash atributes)
There's a lot more to it but this are the basics I believe.