TTC & failing

I am a stepmom to 2 awesome kids (M10 & F7). My husband was reluctant to add another to the mix but finally told me he was ready to start trying for our own little addition to the family (this would be my first bio). Being a mom is all I have ever wanted, so naturally that was very exciting for me. We have been TTC for 3 months now, and have failed every time. I am heartbroken. Both of his kids with his ex wife came quickly and easily, so I feel like I am the one that’s failing us. It’s so hard to be grateful for my stepkids and the life I have when this is something I want more than anything. I find myself drawing away from them because every second I spend with them reminds me of what I don’t have. My husband doesn’t understand the heartbreak I feel every month when I get my period. Has anyone else experienced this? What can I do to get through and not let my disappointment consume me constantly?