Even though I've now successfully had two kids, I have been where you are. Both of my kids are fertility babies. I understand the depression aggravated by the milestones that mean something to you but no one else sees as out of the ordinary. On one occasion, I was once again told fertility treatment had failed the very same day I had an Ethics class that gave abortion statistics. I wish with all my heart I could offer you a miracle and a miracle cure.
One thing I tried at one point when I took a break from the treatments is I spent some time seeing a psychologist. She had not been where I was but had ideas on tools to help me move forward. It also can help to talk to someone who doesn't know you or isn't related to you. Even my DM thought she was being helpful, well-meaning...I not really sure what she thought when she kept telling me to ignore the specialist and just relax.
We were even foster parents to teens for awhile--we stopped when I decided to try one last time to get pregnant. And then when it worked, we did not want to treat our biological child different from other children so we put off getting back into foster care. We still haven't because we returned to the specialist and I didn't want to go through a pregnancy and try to have more than 1 other child at the time. We do have the plans but no specified date to get back into foster care. So I do understand how you can love and take care of foster/adopted children, but there is something that is just different when it's your child.
Realize that it's ok to get upset and depressed, just be careful that it doesn't overwhelm you. Your kids are lucky that you recognize that this bothers you because too many people deny their feelings and tend to lash out those closest to them. Be open with your DH and your doctors. They can help point you in coping directions and be a phenomenal support system. If they are not being supportive, find a new doctor.
If you want to talk more, pm me...my heart truly goes out to you.