Hubby unable to get it on.

I love my hubby and I would anything for him. So here goes. My hubby and I have been married for a year and a half now. We use to get it on every other day. Lately he begins to get soft as soon as he puts it in. I feel so bad thinking it's my fault. The doctor has checked his heart that is okay, checked the blood flow that is okay. Which leaves me to feel it is my fault even though he tries to reassure me that it isn't. Any ideas on how I can help him keep it up? We are trying for a baby and can't get pregnant if he can't stay hard.