We also felt that we'd like to birth a child then adopt. When I'd had two early m/c and an ectopic, I felt so, so broken that I wasn't sure how much more I could do. I hated my body, I hated not knowing why I couldn't stay pregnant, I was tired of spending so much money for what felt like, at the time, another chance at a miscarriage. I wanted to be a mom and was actually never too fussed about a genetic connection. So I was ready to move to adoption first, but we agreed to try 2 more times (we are same-sex so "trying" for us was a little more involved). A that point, we started researching agencies etc so that when we got that last negative, we had basically already been to their info session and were ready to dive in.
I was first to move to adoption, but once we got there, I pulled back and had more reservations that I had to work through. We kept on with the process because I knew it was a long one and because we are queer I thought it would take us a while to get matched. I worked through what I needed to before we finished our homestudy though.
Basically, I was tired of trying. I was tied of hating my body, I was tired of trying crazy gluten free anti inflamatory diets that didn't help, I was tired of feeling broken and isolated. I was tired of fighting myself. I was ready for the process to NOT be about my body's particular ability to get or stay pregnant. I was to enter a process that could be more evenly shared with my partner.
Honestly though I had fears until our son's adoption was finalized. Heck I think I still have some. But that turning point that you mention was pretty clear to me.
Also, although I originally wanted a family by birth AND adoption, I've since changed my mind and if we do add any kids int he future, TTC wouldn't even cross my mind.