I'v been feeling very overwhelmed with my 6 month old going for op in 2 weeks time. Need something to take to calm down when need it. What can I use? Still breastfeeding her but she is on solids also. Must be safe for her.

Anxiety. I would suggest OTC rescue remedy tabs.Comes in drops for children as well.I understand your anxiety,but keep in mind you cannot determine outcome.Stay away from caffeine for now.
Calm. U can use a.Vogel neuroforce drops it is herbal.
Meds/breast feeding. Medication for anxiety as well as most other medications are passed into breast milk, but in very low concentrations, but to be completely "safe", you might consider stopping breast feeding until the operation is over. Consult your healthcare provider for the appropriate medication.