Up and down and cant cope, I feel guilty...

Im now 28 weeks pregnant and have been on anti depressants since I was 4 weeks pregnant.
I feel guilty as having your first child with someone you love should be a happy time and it hasnt been.
I have no explantion for why I feel this way.
My baby was planned and im happy to be having her i love her so much already so why am i so down?
My partner has wanted children for a long time and I feel like ive ruined his first experience of pregnancy.
Some days im great and other days I make our lives a misery. If i feel this way now im so worried that I will
have post natal depression aswell. I suffered from depression before but overcame it, my midwife explained that the hormones
of pregnancy can make your depression etc resurface. I hope to have more children in the future but will my experience of
pregnancy always be this way?
I know alot of mums they always spoke of morning sickness, stretch marks and so on but not one spoke of any depression