GABA is the chemical the Benzos (ativan, valium, xanax etc.) work on. You can buy it as a supplement by itself or in combination with some others like B-6.
Speaking of B-6, how is your B intake? B vitamins are the feel-good, relaxers, yet energy producing vitamins. They also help process sugars and regulate blood sugar, so if that's a factor they'd help. Don't overdo it though, they do have toxic levels, if you're not still taking a pre-natal, you could safely do a B-100 daily (assuming you're not getting any other b's), otherwise a b-50 would be better.
Rescue remedy, or another of the Bach floral essences that match your mood would be safe
Calm's forte is a homeopathic remedy that might help.
I 2nd the Mood Cure, and the suggestion of eliminating all refined sugars and carbohydrates from your diet. You likely are craving them because you're nursing, but if you're having anxiety they are not your friend.
How are you sleeping? Melatonin has been studied in infants as young as 6 months and has been found safe. Sufficient sleep alone can do wonders and 2mg is all I need to fall asleep.
Lastly, a valium or an ativan every now and again isn't going to harm the baby. Your baby needs a healthy mom, if you're really on edge, you need to take care of yourself. You might not want to do it daily, but if you really need to, it's okay.