This is apparently free floating anxiety which in aggravation causes feeling of impending doom where you feel that you might even die due to breathlessness.
These are typical of Panic attack disorder.
This is more of pre-occupation with lung disease which need immediate intervention.
Next Steps
do consult a psychiatrist to start with anti-anxiety medications.
Health Tips
you require assessment with Hamilton depression and anxiety rating scale to score the severity of present symptoms. Also psycho-education about anxiety coping techniques.