Meditation is the Key to Less Stress
I've been meditating off and on for 20+ years since my old masseuse (long since retired) introduced me to it. Breathing is definitely "a" if not "the" key to mediation (not to mention every sport I've done).
I'm definitely of the "sex and sleep only in the bedroom" persuasion. Regarding sleep, concentrating on your breathing ("meditating") brings you into the "now" and away from both the past and the future. Concentrating on the future during a global pandemic and economic downturn is almost guaranteed to keep you awake!
Many meditation trainers say you should breathe in through your nose and out through your mouth. I only did this while first learning to control and become aware of my breathing. I don't like it because it dries my big ass lips out.
Guided meditation is nice, but you most assuredly don't need to pay monthly for an app. Learn what you need online; find what works for you; then go further on your own journey.
Definitely, sex gets better. ;-)