How To Increase Velocity On A Specifically Defined Direction

Hey, currently I'm trying to set a velocity in a specific direction that I defined. The direction I'm using is based on the trigonometry concept of a unit circle. This is how I calculated the direction I want to define.

Code (CSharp):

1.  
2.         XRight = Mathf.Cos(GroundAngle);
3.         YRight = Mathf.Sin(GroundAngle);
4.  
5.         XLeft = XRight * -1;
6.         YLeft = YRight * -1;
7.  
8.         RightDir = new Vector2(XRight, YRight);
9.         LeftDir = new Vector2(XLeft, YLeft);

GroundAngle is the angle that the player character is currently tilted on. Using this angle I made two vector2 that are supposed to act as the "new" left and right directions relative to the player's angle. Using these new direction definitions, how do I go about adding a velocity only to the defined direction? I've tried a few solutions including using "AddForce" on the rigidbody, but it doesn't produce the results I'm looking for. "AddForce" seems to behave differently than "velocity".