No two depressions are exactly the same. It's complicated. You have a lot more trauma than me and, like you, I have never been isolated like many at this forum are. You have a very complicated case; layers and layers causing confusion about even what the best treatment for you would be.
Recovery is possible. I am so grateful (and hope everyone here can experience this) that I am definitely not feeling much depression lately. For instance, I felt so much shame about attempting that the thought of applying for jobs (I have mostly worked my whole life since I was 15 years old) gave me this dreadful feeling about the thought of explaining my time out of the workforce. I am no longer worried about that -- I have no problem withholding it (it is even my right!). When I was depressed, I just couldn't look at my situation this way. So remember that depression "colors our world-(something sophiesmom once told me)" and, no matter what caused it--it is no fun/terrible/sometimes crippling for all of us who have it or have experienced it. I hated it and am so grateful mine is a little better right now. Feeling something is not a right, it just is and we understand.
PS As for feeling guilty about not being good enough for your fiance--you will have a lifetime to show your appreciation. Little things sometimes mean a lot. So does caring enough to seek treatment for your mental and physical illnesses. When it is hard to get out of bed and go to an appointment, know that this is showing how much he means to you. And trust, tenderness, appreciation, affection, kindness, loyalty and understanding can mean so much more than material things....