Frustrated & tired (both mom & baby)

My baby is 18 days old... I feel like we were off to a good start. And when we brought him home, my husband and I fell into a nice rhythm. However, over the past 4 days, my baby just seems to cry and cry and cry. It really makes me question what I'm doing - or what I'm doing wrong.

All his needs are met - he will be fully fed, burped, diaper changed, swaddled, etc. Yet, he gets into these hysterical fits where he cannot be soothed. He'll even fall asleep for approximately 2 minutes only to wake up crying again. It seems that all his 'wake time' is dominated by his crying and it's getting to me... today I just felt like a failure and had to hand the baby off to my husband and take a quick break for my own sanity.

My husband keeps reassuring me that everything's ok. Maybe I had unrealistic expectations of a 'happy baby' - but at this point I'm just so exhausted. I feel like I'm not doing enough for my baby - and then he gets so worked up and ends up overly tired (a vicious cycle).