Baby prefers formula over EBM

I have a large store of expressed breast milk as baby was premature. Baby has always been on the bottle. I started giving formula about 7 weeks ago as she wasn’t gaining enough weight. Now she refuses the EBM and will only take the formula. I’ve tried mixing it; her bottles are 5oz so I do 2oz EBM and 3oz formula and she still refuses that. There’s a lot of back arching and pushing the bottle out of her mouth.

Has anyone experienced the same? How can I encourage her to take the EBM? We have a whole freezer full of it.

Thank you for any advice and guidance ladies