I haven't been posting much about this because I didn't want to share really, but I thought if it helps anyone else, it might be worth it.
I had the same problem during DD's pregnancy - depression. It only happens in 3rd trimester for me, but it can be completely disabling. I have lots of theories about it, but not worth sharing unless someone else is suffering. Mostly, I want to share in case anyone else is dealing with it. It is normal and it can go away. PPD is well-known and much more common, but it is awful to feel depressed during pregnancy as well. For me, I feel both physically and mentally exhausted. I have sobbing sprees that come out of no where (can be really embarrassing). It is hard to explain and feels very distressing. I'm sure at times it is hard for DD and DH. I know every pregnant woman has mood swings, this is just more severe.
Anyway, don't feel like you need to respond. I'm having a really good day today, actually. I'm sharing in case anyone else is feeling really down and may be going through the same thing. If so, feel free to PM me.