Breastfeeding & topping up with formula ** long post alert*

I am an anxious mother and my head takes over at times.. my baby boy is 6 weeks and he has been breastfed and then topped up with formula since he was 3 days old. Little context behind this - we were still in hospital and they weighed him and advised he had lost 9.2% so I was made to feed every 3 hours and pump so that he could be topped up after being in breast. (Pumping is my worst nightmare and it drained me as all I did that day was feed and pump it got to me mentally) early hours of morning I was drained as was baby and he was crying intensely I gave in and decided to use formula as a top up. He gained weight after few days and we were discharged. I have since continued with offering both breasts and then topping up thereafter with formula, he started on 90 and now on 120. My question is everyone time he breastfeeds (spending on average 30 mins each side) he is crying for more milk. Why is this? It’s making me feel like he isn’t getting enough milk from my breast, it’s playing on my mind and making me feel useless. When he isn’t sucking my breast properly I am tending to get lumps and they kill , iv spoken to my nursery nurse support work and she said to continue bf as much as I can to avoid mastitis. I’m at my wits end I don’t know whether to give up breastfeeding and make the full switch to formula. Looking for advice and thanks for reading!