Baby gaining weight but not enough

Im not sure what to do at this point. My baby is gaining weight but not enough. Im not sure if thats normal or what. I have a doctors appointment scheduled for Monday to talk to his doctor. I feel like a failure as a mother. He is only 6 weeks old and my frist. He was 6.1 oz when born and he came at 37 week 3 weeks early. His lowest was 5.6 oz. He is now 8.1 oz. Iam doing both formula and breastfeeding. I have a low milk supply and currently in the process of trying to get that up. Anyone been through this? Any advice will be greatly appreciated.