Breast milk to Formula - help!!

I have breastfed my son for just over 9 months now and am more than ready to stop and have become very negative towards it as it means I never get a break! My son won’t drink from a bottle so I have been gradually giving him formula in his sippy cup mixed with breast milk I tried 2 different formulas now but it seems like he is having a bad reaction from it as he wakes up screaming in pain in the night which is not normal and his stool has been either very solid or very runny - not sure whether to change his formula again or just give up and carry on breastfeeding 🤕 I breast feed him once in the morning and once before bed to relieve my breasts but don’t feel like that is enough milk for him to last throughout the day x