First trimester depression. Help.

I'm 8 weeks pregnant with my 3rd child. With my girls, I experienced a little bit of nausea, but nothing crazy. A little moody, but again, nothing extreme. This time.. The nausea and vomiting is insane– all day, every day. This morning while in the shower, I just lost it. I couldn't stop crying. I'm no longer excited for this pregnancy at all, and I feel terrible saying that, but I feel completely helpless. It's not the feeling sick that's making me feel this way, just an added annoyance. I am second guessing everything and if I'm capable of being a good mother to 3 children, the thought is so overbearing that it brings me to tears every time. After I drop my 6 yo daughter off at school, I'm basically a zombie and lay on the couch all day while my 13 month old plays. I take a break to feed her and change her, and go back to couch. The dishes pile up and I feel like there's so much that needs to be done, but every little task seems overwhelming. I feel like a horrible mother and wife, and I'm finding it so hard to cope.. I'm incredibly depressed. And I already take 50 mg of Zoloft, I have been since I had my last daughter.. Is there anyone who is going through or has gone through this? What did you do to get through it? Anything, any advice is appreciated..