diminishing milk supply

I used to pump 30 oz a day which was a bottle extra what my son ate every day. It was nice building a stash. Now, I went back to work and I only pump inconsistent amounts which causes me to dip into my stash. Which I hate because I am almost out and I wanted to stop breast feeding early and just use my stash. I am now pumping what he eats next and it's so stressful. I use fenugreek, I try and eat extra calories. I know water is huge and I have been slacking but I barely make 24 oz now and he drinks like 28 some days. Should I buy formula to feed him for that last feed that I am not making? I would still pump just as much and try to pump more. I would rather feed him than my fridge but it stresses me out that I have nothing saved just in case. What should I do?