The goal of this post may not have been a "critique my stroke" but your head is too high when it is in the water which is probably why you "sink like stone". Seriously this isn't a classic your seat is too high response, get more of your head down into the water (it seems like you are trying to look too far forward and not enough down) and it will pull your legs up. I'm willing to bet your neck gets a bit tired or sore, it is from trying to hold your head up. FYI - I'm not a coach or swim expert, but that is what my eyes are telling me.