breastfeeding problems

Jack has both ult and ptt and breastfeeding is becoming harder and harder. Feeds either take ages or they take 10 min because he falls asleep tired but wakes up 30 minutes later hungry again. My nipples are purple and bruised from the constant frustrating chews and slaps he gives me. I've tried expressing but i keep stressing about him not gaining weight properly. We have an appointment in 2 weeks to have it snipped but i don't know how I'm going to keep going like this :(