I need advice, please help!

Me and my partner have been together over a year and half and we have a baby together he is now 12 weeks old I’m also a first time mum. Lately we have been fighting about small things and a few big things. I was breast feeding my baby until 11 weeks when I notice he was hungry all the time and wasn’t getting enough milk so I put the baby on to formula milk he’s been on the milk for a week now and he is gaining weight beautifully and In my head I know he’s getting enough and I can monitor how much he drinks however my partner did not agree to this and things I’m being selfish for putting him on the formula milk. Another one of our fights are about his other daughter who doesn’t live with us and he sees her every other weekend when I was pregnant and he told his baby mum she threatened to slap me once I had the baby I’ve met the little girl twice and Im worried he’s not gonna have a bond with our new born like he has with his other daughter. He’s not helping me with feeds or changes his nappy I just don’t know what to do anymore. Please can I have some advice? Thank you in advance