Help with locking mouse in ONLY X axis?

So I'm making this game and I'm trying to make it first person where you can still move the mouse up and down. I have created a camera part in the workspace which gives the current camera a position in the world, and the camera follows the mouse, but won't move up or down. I need to lock the mouse in the x position, not the y, to stop it from freaking out. Does anyone know how to do this?

cam = game.Workspace.CurrentCamera
local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()
local val = false
while wait() do
	if game.Workspace.Game.Value == (true) then
		if workspace.Monitor.Value == (false) then
			cam.CameraType = 0
			if val == false then
				cam.CoordinateFrame = CFrame.new(workspace.MainCam.Position)
				val = true
			end
			cam.Focus = CFrame.new(mouse.Hit.p.X, workspace.MainCam.Position.Y, mouse.Hit.p.Z)
		else
			val = false
			cam.CameraType = 0
			cam.CoordinateFrame = CFrame.new(game.Workspace["Cam"..game.Workspace.CameraId.Value].Position)
			cam.Focus = CFrame.new(game.Workspace["Cam"..game.Workspace.CameraId.Value.."Focus"].Position)
		end
	else
		cam.CameraType = 0
		cam.CoordinateFrame = CFrame.new(game.Workspace.MainCam.Position)
		cam.Focus = CFrame.new(workspace.MainCamF.Position)
	end
end