I think I look a little stiff...

[With_Two_Flakes]

Someone earlier had a problem with your lack of movement, but I don't have a problem with you standing still. Main problem I have with officials when I assess them is that they have "happy feet" and move too much when they don't need to. In addition, when they do move - they move using the wrong style of movement. You should try to stay facing the field with your body shape for as long as possible only turning when you need to chase down a long run or pass.

Your initial position is nice and wide on the sideline which is good. The play seems to still be a distance from you (beyond the numbers), so there is no need to move into the offensive backfield for your own safety as you are not in danger. It is coming to the LOS so it is fine to stand still and let it come to you. If the runner makes yards beyond the LOS then stay with him moving with a side-to-side boxer shuffle for as long as you can. Only if he breaks out for long yardage, do you need to turn and pursue.

Whistle in the mouth would get you a ding if I assessed you.
No need for it and it will eventually get you into trouble one day. Try a finger whistle, try a wrist lanyard, try holding it in your hand as suggested earlier - find one of them that works for you as it will save you from an embarrassing IW.

How much room on your sideline do you have? There appear to be folks who might be in your way if that runner made yards. When you move side-to-side, you cannot afford to be looking where you are going, you keep your eyes on the players. Therefore you must be sure there is no-one in your sideline area.