My Step-Daughter is Bi-Polar but the 7 yrs i lived with Her and Her Brother (who is autistic) and Her Mom, i only knew of Her Brother's diagnosis -- You have spilled in sooooo little words exactly what my Step-Daughter was and was Feeling.
Your spill here can / could apply to Her Brother, as well, i think.
I had a relationship with a woman for 6 years on and off. She always claimed she had a chemical in balance on those very dark days. Back then there wasn't a name for it and she suffered mightily.