woke up struggling to breathe in, finally opened my mouth and breathed. Heart was pounding, sweatin

A couple of times recently have awakened from night sleep, unable to breathe, as if a pillow is completely blocking my nose (unlikely).  I finally manage to open my mouth and gulp air.  It's very, very frightening.  My heart pounds and I'm sweating terribly.  I'm 61, female, slightly under normal weight, physically active (do 100 situps every day! and walk daily).  I do not have insurance and can't afford it.  Self-employed, artist.  Am more often than not very irritable, very depressed.  Have had sleep difficulties for years:  getting to sleep, staying asleep, some snoring, waking up too early.  About 3 years ago, had what apparently was a lung infection and had really bad time breathing during the day, fairly low blood oxygen levels, couldn't walk more than a few feet, slept all day.  With time, job change (was a potter and quit it), some medication (prednizone), symptoms went away.  Except for difficulty sleeping and irritability and depression, I'm in very excellent condition.  Highly educated, not a hypochondriac at all, eat well, exercise well, everything's fine except for those 3 symptoms.  But the worst are the frightening episodes when I can't breathe at night.  Oh, and I can't sleep on my back, either:  causes nightmares for some reason, so I sleep on my stomach or side.  That's it.