Do not feel bad for a situation you could not of prevented. I went through exactly the same and thankfully i have been able to provide my milk but its demanding, IMO more demanding or time consuming at least than ebf.
Some tips for upping your supply, Lots of water at least 3litres per day, lots of oats, my diet is mainly porridge, flapjacks and hobnobs and anything that has oats in it lol. You can take fenugreek 6 tablets a day and i was prescribed tablets from the doctor (he was pretty reluctant to give me them at first though) so far its working and ive only resorted to formula for a couple of feeds through the 4month growth spurt.
I just felt frustrated that i had milk but issac couldnt get it himself
Big Hugs xxx