While you are highly stressed it will be difficult. Treat the cause first.
Actually, some feel that the effects of stress can be combated by dealing with the physiological actions as well, that all mindfulness or meditation realty achieves is to improve breathing patterns,but wrap it up an a nice fluffy blanket.
It is a downward spiral, breathing through your mouth doesn’t oxygenate the blood properly, and disturbs your sleep, leading to always feeling tired and unfocused, you don’t have the energy to do the things you like (ie mountain biking), you put on weight, small things become big problems, stress increases, breathing and especially nighttime breathing become worse and it all feeds into the spiral again.
The majority of the population breathe through their noses normally (and many will no doubt just dismiss the potential issues), but a sizable minority are not in that lucky situation and it can certainly have a contributing and accumulative affect on other health issues.
That is not that i am espousing alternative therapy instead of getting proper qualified assistance for stress. in the Steve jobs mould. But breathing patterns, and especially nighttime breathing patterns, are something that medical science is starting to take seriously and is worth exploring alongside.