Been there - done that... With my second pregnancy I was extremely depressed and not on medications - at that time they hadn't decided if any were really that safe (17 years ago) - I made myself seem ancient!
I do understand where you are coming from and for me, I had a lot of support at that time and I was in therapy, which helped me because I was very upset at the fact I was having another boy. (Wouldn't trade my boys for the world - but there was something about having a girl.)
I did a lot of sleeping and crying. Sleeping was very beneficial especially the further along I got. Now the crying... I tried to read books, I did puzzles and I guess one of the best things was spending cuddle time with my 3 year old. You say you have 2 other children - love on them and squeeze them and let them know you are madly in love with them because pretty soon - they might be feeling a little neglected. And by the way - origami is a good thing to keep your mind occupied.
Keep your head up and if you need to talk - let me know. Before you know it that new little one is going to be in your arms and you'll have one more person to fall in love with!
Here's a hug just for you...