How to deal with work stress? ?

Go at a easy pace. Don’t kill yourself, it’s not your company. If the work doesn’t get done, oh well. There’s tomorrow or they can hire more staff. 

You have some good answers here. A really simple but effective thing - the habit of responding to moments of stress by slowing your breath. The evidence showing that slow breathing is a fast and effective method of calming down is enormous. It's used in treating anxiety, depression, and PTSD. Soldiers say that they are able to calm down immediately in dangerous situations by simply slowing down their breathing.

Is the overtime necessary?  If it is, then take small 10-15 minute breaks every hour.  Inform your supervisor that you need that in order to perform well.  If this kind of management continues, they'll lose the two of you, too.

Mind your regimen of daily outdoor time, exercise, eating properly, avoiding intoxication and other vices.  These times demand discipline.  Get to bed early.

Having too much work is better than having no work.  Ask to cut out overtime until you can regroup.