I need to vent...

So, before my fiancé and I met, he was with his ex. And she got pregnant before they broke it off.
Fast forward to today. My fiancé and I have been together for a while now, and ttc. She's a full term now. She's about to have their baby boy any day now. They're currently at her ultrasound and well... Month after month if ttc and I haven't been successful. And it just makes me so damn depressed... Like I'm not doing something right. Idk... I guess that's all...
I support him and will be with him even with baby mama... But I just feel like failing and can't help but feel upset and mad at myself...