When you are stressed, one of the tricks people shared with me is to learn the art of deep breathing.
You see, we humans have shallow breathing, at least most of us only use a small percentage of our lungs capacity to inhale, hardly moving the diaphragm (slightly above your tummy).
If you practice deep breathing a few times, if done correctly, you will automatically yawn. That is how relaxing this action is.
Press your abdomen and ensure it goes all out when you breathe in and all in when you breathe out and repeat.
I as guilty as anyone for not making deep breathing my habit. After a couple of attempts, I am yawning and hoping for a nap!
Anyway, the idea is to relax and don't be too hard on yourself, hence reducing stress, hopefully.