Yoga is a practice. Remember it's a journey in witch consistency and quality are more important than quantity. Having an everyday practice would be far more beneficial for your health and journey.
In fact, while doing downward dog in yoga, first bend your knees and push the floor with your hands as much as you can while bringing your chest closer to your knees. And thereafter explore to straighten your knees. In downward dog the goal is not first having straight legs, rather relaxing the upperside body. Another tip is also taking your belly in thus feeling really the core muscles at the lowerback. That may definitely help for the arching
Try and put you back in a straight line. your hips should be the highest part of your body and your shoulders should be away from your ears. a straight back should help the problem.
I have the same problem ! Check on YouTube for video explaining how to do it. Your wrists have to be well aligned with your shoulders. Thumb and first finger in a L shape. Hands make a kinda succion to the floor. Legs don't need to be straights, heels don't need to be on the floor. Your back should be in the same alignment as if you were sitting straight, arms up.
Well just try, don't force it, don't look at you in the mirror, just imagine the shape and try to do it right. It takes time, but it will be ok, when you practice regularly :).