Feeling like a failure....

My husband and I have been TTC for 3 years almost 4. We have a 6 year son and he desperately wants a sibling. I feel so horrible that I can’t give them and myself what we want. I never planned to have one child. I’m extremely grateful for my son and I love him more than anything but we want more children. I have pretty much given up. I wanted kids close in age too and now that won’t happen. I feel like a failure because almost everything I planned hasn’t happened. I keep trying to accept this is it but then I see babies and I just want another one. Am I a bad person because I can’t just accept this is it? How do I explain to my son he will never have a sibling? I never thought I would have to tell him something like that. He gets so excited to talk about one day he will have a brother or sister and I have to tell him I don’t know if you will and he just looks at me confused. It hurts...