Black Betty bam balam 15 months old

I just brought my betty home from ICU where my vet sent me bc her sugar was 300! They are surprised That she has juvenile diabetes at 15 months old... I used to leave the food out all the time and I still give them wet food every day.. I have 2 other cats.. how do I make all of this work all of them together.. I feel overwhelmed but am ready to tackle this.. I’m hoping For remission!!! How do I make the house hold comfortable for all my babies? She seems depressed and Keeps going to the food bowl .. I don’t know what her sugar is right now.. but the dr. At the hospital said I don’t need to worry about that right now.. just her shots and her meals.. Oh my kids have been in this routine forever and they are hungry too. This is breaking my heart! Just spent over 2 grand so far