Does anyone ever find they feel really down, and then more down because they feel guilty about feeling down??

This is my second pregnancy (I'm 24wks), very much wanted. I had a missed m/c at 12weeks first time around, but fell pregnant again fairly quickly. For some reason, I go through periods of being really depressed and down in the dumps. Most of this seems to centre around my concern about Down Syndrome. I have had u/s and bloods, and my risk is 1:2910, which I know is low risk and ok. We have a child with Cystic Fibrosis in our family, which was a shock and although we wouldn't change him for the world, it has changed lots of things and it has probably affected me more than I had ever realised.

I have tried talking to my ob, my sonographer and my husband, who all tell me I'm low risk, but there is a chance! I feel like none of them listened to me and it's now a touchy subject between me and DH. All my girlfriends appear to be risks of 1:8000 and so, and I can't understand why me, a normally logical and controlled person, who has never suffered with any sort of depression, is struggling so much. I feel like I'm not even paying attention to the kicks as much as I should because I'm too worried.

Have made an appointment with my GP next week, so hopefully she'll listen to me...I almost feel like having an amnio to put my mind at rest, but I know that's not a wise move..

Anyone felt the same, or is it just me being ridiculous?