you poor thing. Pregnancy can be a really hard time for all of us. I wish I could offer some advice on how to make that pesky M/S go away but if I knew the answer to that I'd be a billionare.
I hope your DP gives you a big hug tonight and maybe a nice massage as well.
Although feeling down is part of beiong preggers if it gets to a point where you really feel that you can't cope you should talk to your Dr about it. They might be able to suggest some coping strategies.
Ante-natal depression can be very difficult to deal with and you shouldn't have to struggle on alone.