The pressure to get my wife pregnant is causing me to not perform

Donโ€™t know really where to start and what to say. But the problem has started to occur when my wife and I have decided to try and have kids. Prior to her ovulation window we could have sex as normal but once she was close to the ovulation window I get stuck in my head. The pressure to perform would make me lose my erection and unable to ejaculate. She was really understanding and tried to keep me relaxed and focused throughout. After a few months I started to get a bit better and we managed to get pregnant. However, we then had a miscarriage and it has become so much worse. We have been trying again and have not been having any luck for many months. Every month that goes by it becomes more difficult for me to get erect. I feel like I am letting my wife down and that i am the reason we canโ€™t get pregnant again. I am so much in my head, that i fear to try having sex because i can see the pain in her eyes everytime I have sex and cant perform. She is being very understanding but i know i am letting her down.