To drink or not to drink formula

Pea doesn't like to drink formula. Whenever we feed her formula, she'll drink lil' bit, cry, drink, cry and on it goes... and that also she absolutely won't finish the milk (unless she's sleepy)... But yesterday after hubby fed her formula and she did the same old trick, it makes me wonder whether does she only like drinking breastmilk. I've read that the breastmilk is somewhat sweet... is it true ?

If Pea continues like this, I'll have no choice to be exclusively breastfeed her. Not that I don't want to exclusively breastfeed her, but it's just that I'm the lazy type. Previously (when we first had Pea), I would wake up around 1 / 2 am to feed Pea her formula milk and then express out my breastmilk for her next feed (which was around 3 / 4 am). While hubby takes over the next feed, I would be expressing milk again (instead of sleeping). Initially when I tried breastfeeding Pea, she took ages to finish (could be around 1/2 hour ?); that's the reason I used that method (feed formula, express milk for next feeding)... With this, Pea has one time of formula, and the rest breastmilk.

But now, I guess I've been slightly more hardworking, thus I'll feed Pea direct and then only express milk out (she'll drink around 10 mins on one side and then will go back to sleeep). There are EBM stored in the freezer and fridge, but I wonder how long those will lasts ? MIL expected me to finish using them up before I start work... her thinking is that when I start work, I'll be expressing again, thus the expressed ones can let Pea drink the next day. My aim is to let Pea have breastmilk as long as I can take it (that means I'm not stressed up to the maximum... I wouldn't know as I've yet to start work), and that I can still express milk out (there would be times when I've gotta go customer's site and that depends whether they have a suitable place for me to express milk).