Why not try allowing yourself to feel this? It is perfectly OK to be very very sad and also even angry that this has happened to you. Never mind that other people have much worse things happen to them.
I too have secondary infertility and have one DS. He also has a diagnosis of ADHD and ASD so I don't even have the experience of raising one "normal " child (whatever that means!). DS is now 9 years old and I still occasionally fall into pools of real sadness and despair as to how my life turned out. Overall though I have come through the really bad times of grieving for him being an only and also for his diagnosis. Mostly now I just love him and make the most of what we have.
Do get your hormones checked out as has been suggested and look into things like mindfulness. I have found this quite useful to help me gain a different perspective on things. You know life is not always easy for a lot of people but really what makes us who we are is how we take the things that happen to us and how we let them shape us.
You have one child and from what you say will be unable to change that, but how you live your life with one child will affect how that child grows up and the relationship you have with him long term.
The serenity prayer is really really apt for things like this, it really is. You don't need to believe in God, just the sentiment
"give me the serenity to accept the things I cannot change, the courage to change those I can and the wisdom to know the difference"
You can't change that you only have one child but with some courage you can change many other things about your life. You really can. At the same time allow yourself to grieve for what you couldn't have. You can grieve and be kind to yourself and be brave as well. All of these are possible at the same time, they really are.