What's the issue?

You're only applying the force to the X axis. You need to equally exert force on all 3 axi.

lookVector

lookVector, a property of CFrame, can return the forward direction that a part is facing. It returns a 1 if it's fully facing an axis or -1 if they're facing the negative side of the axis and it can also return all the numbers in between. It can do this to any axis.

print(workspace.Part.CFrame.lookVector)

1,0,0.84

This part is facing towards the positive X and a little bit towards the positive Z axis.

Final Product

We can do arithmetic with this. I made a new variable called BoostForce which is the max velocity something can move in all directions. In this case I made it 50, you can edit it. I multiplied BoostForce with Vector3.new(1,1,1) to get Vector3.new(50,50,50). Then I multiplied that with the lookvector to get a percentage(1 is 100%, 0.84 is 84% and etc...). If I were to use the part from the last chapter I can multiply the last part's lookVector with Vector3.new(50,50,50) to get: Vector3.new(50,0,42) which is what the part's velocity will be.

    local plyr = game.Players.LocalPlayer
    local BoostForce = 50 --50 studs/sec; changeable

    while true do
    db=false
    j=0
    repeat wait(.1) until plyr.Character
    local InputKey = "ButtonB"
    deb=false
    B1=false
    B=0

    local UserInputService = game:GetService("UserInputService")

    UserInputService.InputBegan:connect(function(UserInput)
    if  UserInput.UserInputType == Enum.UserInputType.Gamepad1  then

            if UserInput.KeyCode == Enum.KeyCode[InputKey] and deb==false and B1 == false then
    B=1
    for i = 2,1,-1
    do

    print (B)

    deb=true

    B1 = true

    if  UserInput.UserInputType == Enum.UserInputType.Gamepad1  then if UserInput.KeyCode == Enum.KeyCode[InputKey] and deb==true and B1 == true and i>0 and B==1 then


    plyr.Character.Torso.Velocity=Vector3.new(1,1,1)*BoostForce*plyr.Character.Torso.CFrame.lookVector
--Instead of Vector3.new(1,1,1)*BoostForce you can also do Vector3.new(BoostForce,BoostForce,BoostForce)
    script.S2:Play()

    local character = plyr.Character

    local animation = character.Humanoid:LoadAnimation(script.Slide)

    animation:Play()

    wait(.2)

    B=2

    print(B)

    deb=false

    wait(.5)

    animation:Stop()

    else
        B=0
        B1=false
        deb=false
    end
    end
end

        end
    end
end)

end

Hope it helps!

Edit:

You can just multiply the lookVector and the BoostForce together only to make it shorter.

plyr.Character.Torso.Velocity=BoostForce*plyr.Character.Torso.CFrame.lookVector

You may use that line instead of the longer one.

    local plyr = game.Players.LocalPlayer

    while true do
    db=false
    j=0
    repeat wait(.1) until plyr.Character
    local InputKey = "ButtonB"
    deb=false
    B1=false
    B=0

    local UserInputService = game:GetService("UserInputService")

    UserInputService.InputBegan:connect(function(UserInput)
    if  UserInput.UserInputType == Enum.UserInputType.Gamepad1  then

            if UserInput.KeyCode == Enum.KeyCode[InputKey] and deb==false and B1 == false then
    B=1
    for i = 2,1,-1
    do

    print (B)

    deb=true

    B1 = true

    if  UserInput.UserInputType == Enum.UserInputType.Gamepad1  then if UserInput.KeyCode == Enum.KeyCode[InputKey] and deb==true and B1 == true and i>0 and B==1 then


    plyr.Character.Torso.Velocity=Vector3.new(50,0,0)

    script.S2:Play()

    local character = plyr.Character

    local animation = character.Humanoid:LoadAnimation(script.Slide)

    animation:Play()

    wait(.2)

    B=2

    print(B)

    deb=false

    wait(.5)

    animation:Stop()

    else
        B=0
        B1=false
        deb=false
    end
    end
end

        end
    end
end)

end