While I think it's perfectly normal for a longterm relationship to ebb and flow, I don't think your thoughts about driving into pylons are. I agree that you should talk to a professional. Pronto.
I found my therapist through a friend. Do you know anyone who sees a counselor or social worker? If you don't, then check to see if your insurance company--if you have insurance--has a preferred provider. Make sure in making the appointment that you make it clear that you're having some dark thoughts. The therapist will likely try to get you in sooner rather than later.
I've had several bouts of depression. They're downright scary. Right now, I think you should remember that depression colors EVERYTHING. It could be that your marriage is contributing to the depression, but I would stop short of placing too much blame on it until your head is a little clearer. I do think, however, that you should try to be honest with him that you've been down and that you need to get some help.
Like Oakleaf, I think you need to be healthy on your own before you can really address what, if anything, is ailing your relationship. Hopefully, your DH will be open to the process that therapy entails.
Hugs and good luck to you.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher