Finding it hard 😩

I'm 22 and my partner is 28, we've had 3 miscarriages and have now been TTC again for 6 months. He has a 5 year old daughter from a one night stand when he was younger. I know this sounds so bad but I can't help but resent her. I want a family with my partner so badly and it's so so hard seeing him be a daddy to somebody else's child. I know it sounds selfish and I feel awful for having these feelings but I just can't help it. Even worse his ex-wife is now pregnant again (by accident) and I'm desperately trying to have my first baby. How do people get pregnant from 1 time and no matter how hard I try I'm still not a mum 😩