Straight back, and hinge at the hips. You're a thin guy. You should be able to do that, but I'm not seeing it. I'm seeing hinging in the back at the first dotted white section on the rear of the jersey. It's not hard. Sit in a chair with your various joint angles at 90ish degrees. Upper legs and upper arms should be around parallel to the floor. Lower legs, torso and lower arms should be vertical and at right angles to the other stuff. Now hinge at the hips and lower yourself into what would be a riding position keeping your back straight. Now do the same for your lower arms and lower them onto what would be imaginary hoods. There you go. That is the feeling of what your riding position should be. Make adjustments from there. Of course when you correct this, you will find out you need a longer stem or a bigger bike. As for neck pain, I have the annoying habit(like Chris Froome) of constantly switching between looking down the road and looking at my front hub. I make it worse by going from the front hub to the crank/BB area the to the rear cluster area and repeating the whole process over and over. It does cause neck pain on long rides. I'm trying to correct it. Maybe you are doing something similar, Constantly checking the areas of the bike.
