It's natural to feel overwhelmed (or like the worst mommy on earth) if your newborn seems like she screams all the time, especially when nothing you do seems to comfort her. First, try to get a handle on why your baby may be crying. After checking for the obvious (hunger, poop, sleep), know that some infants are simply more sensitive to the sights and sounds of the world around them and crying is their way of letting you know they're distressed. If you can identify what specific stimuli may be triggering the meltdowns, you can tweak the environment to make your baby more comfortable. Is she sensitive to loud noises or too much light? Is she uneasy around strangers? She might even feel overwhelmed by too many toys around her nursery.
If making subtle changes to your baby's surroundings doesn't help, look at the number of hours she spends crying. It's considered excessive for a baby to cry for three hours or more a day for three or more days a week. If this is the case, talk to your pediatrician -- your baby may have colic (which usually subsides by 3 or 4 months) or bad acid reflux, which can be treated with medication.
Recognize that adjusting to a new baby -- especially a fussy one -- can be hard, particularly for first-timers, so it's important to take care of yourself too. This means accepting offers of help and getting as much rest as you can. Try to carve out some alone time whenever possible (even if it's only an hour) to take a walk, browse in a quiet bookstore, or meet a friend for coffee. If you find yourself becoming anxious during a crying spell, it's okay to put your baby down in a safe place (like a crib or swing) for a few minutes while you collect yourself. Remember, even at this tender age, your baby picks up on your tension, and it will be much harder to soothe her if you aren't calm yourself.