I have to actively try to breathe through my nose, it's often congested (having a pet despite allergies doesn't help), and it's just simpler to breathe through my mouth, cause that just works.

Medical interventions are possible, but I'm not thrilled with the side effects of the medicines I've tried, and surgery has some really awful tail risks, so there's that. I was recently recomended to try B1 (thiamine) suppliments related to sleep apnea, not sure if it helps with that, but it does seem to open up my nasal passages, and taking it before bed seems to be helpful with breathing during sleep. YMMV, not medical advice, etc.