Depression is such a complex problem and unique for each individual.
Firt off, good quality sleep is the single most important factor in emotional well-being. Theta, delta, and REM in roughly thirty minute stints with five or six total cycles of 90 minutes each and every night being ideal. Any disruption of that will have cumulative negative effects on your emotional well being.
Fibromyalgia impacts sleep negatively. Non trad meds taken at night might also do that. Pain medications can disrupt sleep as does pain, as you well know.
Heart problems cause anxiety. Anxiety disrupts sleep. Sleep apnea must be treated if it is evident or depression will persists no matter what meds are taken. Beta blockers like inderal contribute to depression. Any persistent low level lack of oxygen can eventually create depression.
Then there are the life issues. Fear of death, fear of death with unfinished business, loneliness, sadness over attenuation of emotional ties, losses we face as we age; jobs, health, people, pets, status, finances,dignity, unrealized dreams. There is so much that can make us sad and depressed.
Keep working on your health and adequate sleep Katie. Go to the therapist, come to terms with losses. Enjoy the present; your friends, family, grandchildren. Get lots of sunshine and fresh air. Enjoy nature Write, you love that I know. Tell your story, the painful parts as well as the bright parts. Good luck..You sound like you're better than you were in Spring.