Depression

I am always really depressed and sad & it really sucks because this is supposed to be a joyous time! Does anyone have any advice for me? I feel overwhelmed, anxious and scared. Our pregnancy was a total surprise and my fiancé is so excited to have this baby, but I'm absolutely petrified but want to be excited for him. If anyone has advice for a new mommy please let me know. Also, for the constant moodiness. I am at my second trimester now and I feel more moody than ever! HELP!