Give the formula straight in separate bottles. That way if baby doesn't finish a bottle you won't be wasting breastmilk. Formula has to be thrown out if it isn't drunk soon, but breastmilk can sit for a bit and you can finish the bottle later.
You'll be fine, mama. I know it's ideal to give only breastmilk, but giving formula while you're at work isn't a failure or an end to your nursing relationship. I had to use formula at day care with both my kids and we were able to maintain wonderful nursing relationships when we were together.
“We are not put on earth for ourselves, but are placed here for each other. If you are there always for others, then in time of need, someone will be there for you.”