This was my life for two years with my middle child. She would cry - not a baby cry but a gut wrenching wail - for HOURS. Until she was hoarse. Nothing would soothe her. Medical advice was to let her "cry it out". My inner-mama KNEW that something was wrong. But I had no resources.
That said, sleeping through the night isn't the be all, end all. If everyone is getting enough sleep, it doesn't matter if it's all at night (just yet).
If your mood is suffering, make sure you are still taking your prenatal vitamins. Make sure you are getting adequate nutrition. If YOU aren't at your best, it makes it that much harder to cope. Post-partum depression IS a valid diagnosis for the first 12 months.
I highly recommend "Sleepless In America" by Mary Sheedy Kurcinka. I recently finished reading "Raising Your Spirited Child" by the same author and it really opened my eyes - especially the chapter on sleep (or lack thereof) - to the issues we had previously had.
I really feel for you. My first child slept through the night by 8 weeks. And has ALWAYS been an excellent sleeper except around 18-24 months when she developed "night terrors".
If you can't get a copy of "Sleepless In America", try to grab a copy of "Raising Your Spirited Child" and read the chapter on sleep. There could be a variety of reasons why your infant won't sleep. They could be too hot, too cold, to constricted, not constricted enough, maybe there's a random noise that is awakening them. If your infant is as sensitive to EVERYTHING as my middle child is, it could be the hum of the refrigerator. For TWO YEARS, we couldn't flush the toilet after she went to bed because the flush would wake her up.