Baby do not accept feeding bottle & Nipple

I need help... 
My baby girl is 2 months now. I feed her breastfeeding only in this period. I want to change formula bcoz I have to go back to work at October. 
But my girl do not accept feeding bottle and fake nipple. I try and try, again and again.. But she refuse to drink and badly crying.. She don't drink and fall asleep. I cry too..
And then I feel sorry.. After that I feed my breastfeeding to her.. She drink with happy face. 
But I must change from breastfeeding to feeding bottle before I go back to work. 
Please give me an idea how should I do.