Need help deciding

I mainly breastfeed now, but I'm having some issues. She's a preemie and gets tired easy or too weak to even try to latch. So, she gets regular bottles. At least two a day and sometimes she only latches once a day. My problem is that I'm pumping less and less everyday. I can always hand express so I know I'm not dry, so I know I have supply to still latch her. Should I just use up my freezer stash or 50/50 with formula to make my milk last as long as possible?