is it okay to formula feed while your breastmilk comes in ? makes me feel awful when my baby isn't full & wants to eat…

you can but nurse as much as possible, the stimulation will help your milk come in and help to establish a good supply. Keep in mind their stomachs are very very small and colostrum is very nourishing and filling, most of the time babies don't need to have formula before your milk comes in.

if you do I would suggest using a dropper while you nurse rather than a bottle because very young babies sometimes have a hard time going from bottle to breast.

You probably don't need it, she is still super young! But do what feels right to you.