Baby prefers formula now :(

Brandi • Expecting baby #1 with my wonderful husband :) loving life!
So my daughter is 4 months today, and for the last month or so we've been supplementing her breastfeeding sessions with 2oz of formula due to slow weight gain. All was going well but the last week she has refused to nurse unless extremely tired or hungry and even then she will fight me for almost a half hour before finally feeding. 
We decided to switch to pumping and just feeding her out of bottles as we were both ending up in tears, but now she won't even take the breast milk out of a bottle but she gobbles the formula right up! 
I think she's decided she's done with breast milk, I'm just feeling a little sad about it 😢 I wasn't ready yet to stop breastfeeding!