Awww Carlie, I really feel for you, I'm 24 and have an 11 month old baby girl. Me and my partner went through the exact same thing you are describing.
I believed I didn't love him anymore, I thought he was useless with the baby, his family really scared me - I hated them being anywhere near her, they just seemed to do/say/think the exact opposite of what I wanted for my daughter.
I eventually spoke to my HV who diagnosed Post Natal Depression - just knowing that there was a problem and that I wasn't a bad person, or losing my mind seemed to help. I never sought any treatment for the PND, which I regret, but in the last 2 months feel as if I am getting back to 'normal'.
Please find someone to talk to about all of this, preferably someone impartial who won't just slag off your fiance.
You need some support! Maybe getting your HV ar GP involved will show your partner that there is a problem, and then you need to sit down with no distractions and calmly and honestly, lay all your cards on the table.
This is what we did, I realised that my daughter could not be my life because one day she will have a life of her own. I also told my partner how I felt about him, which was hard to hear but he realised how serious it had all got and began to make an effort, he now understands that this is his family - me, him and the baby. She is not just my baby, and he needs to do his fair share. He now tells his parents if they're doing or saying something which I feel is inappropriate.
Sorry for rambling, but I really do hope you get this sorted!
x