Hi, S.,
Deep breath. Your stress can negatively affect your production. If your baby is happy for the sitter and is showing appropriate signs of growth (as other moms have mentioned), then she is getting enough! :)
I have successfully BF 2 babies for about 12-13 months each. Getting ready to have baby #3, so I am gearing up to try this again! I understand how stressful it can be to pump at work and to constantly worry that you're not going to have enough milk for your baby. Here are some things you might consider:
First of all, I would not pump every 2 hours at work. Make sure you're getting plenty of fluids, and maybe stretch it to more like every 3 hours.
Second of all, when you're not at work, nurse the baby. It's not always possible, but try to avoid bottle feedings outside of the 8 hour workday. Your baby will help stimulate your milk production better than even the best pump. Plus, she'll get the extra milk that she needs.
Third of all, watch the time of day! Most women produce more milk in the morning, and less milk in the late afternoon. This was definitely the case for me. It made a HUGE difference for me when I started pumping on one side and nursing on the other side at the same time for the FIRST feeding of the day. It was a little challenging at first, but I found that the extra milk I had for that first feeding was plenty to feed baby on one side, AND I would usually be able to pump 5-6 ounces of milk from the other side with just that one feeding. The trick was that with the baby nursing on the other side, the milk would flow (and flow!). As previously mentioned by others, the pump is never as efficient as your baby. Oh, and I should add that I figured this out at a time when I, too, was pumping several times a day at work. I, too, was getting frustrated that I couldn't hardly get any milk during these pumping sessions. I would be lucky to get 4-6 ounces total during the full day's worth of pumping sessions at work. WOW, did it make a huge difference knowing that I was already able to store 5-6 ounces at the beginning of the day with that first feeding, and I didn't feel such huge pressure to produce during the rest of the day. Oh, and on my days off, I could still do that morning pump session while nursing, which would allow me to store up a little extra for the upcoming work week.
As far as the amount of milk needed, every baby is different. At about the 6 month mark, each of my babies were eating between 6-8 ounces of breast milk every 3 hours at the babysitter's. So with 5-6 ounces pumped during the morning sessions on the weekends to use for the upcoming week and a goal of 12-16 ounces pumped per day, things were fine. I didn't have to supplement at all.
The fenugreek tablets are also something that I used at one point. They DO help to increase your milk production. They're easy to get and won't harm the baby. They will make your sweat smell like maple syrup, though! :)
Good luck, and remember that every little bit counts. You are a good momma no matter what. Remove some of that pressure from yourself, and you'll feel a lot better!