please help me....

So, I guess I need advice or support or something, I don't really know.

My son is 8 weeks old today and only weighs 9 and a half pounds. He has reflux so he's an extremely fussy baby as is. I was exclusively breastfeeding but I'm starting to feel like he's not gaining enough weight...so I started giving bottles of formula here & there. And he's already sleeping better. But the idea of not breastfeeding him devastates me. I love it and I love bonding with him. And every woman on my mom's side is very adamant about nursing so it's a lot of negative pressure. I just want him to be happy and healthy. I'm not sure what I'm asking I just know I'm desperate to not be so depressed and lost over what to do

I'm also suffering servere postpartum depression so I'm struggling so bad all together...