piglet, YANBU imo!
Being infertile for a long or short time can make anyone feel deeply irrational.
It took me nearly three years to conceive ds (9) and then nearly 7 yrs to conceive dd (2).
This advice is based on what I went through and what I've learned from other people.
Go for full medical fertility checks asap, both of you.
Give yourself a massive break and don't beat yourself up at all (if you can stop yourself)
Do everything you can to improve your health and fitness etc within reason, you'll feel better too psychologically
Cherish the one you've got when you can
Grieve fully for the one you lost when you're ready (it took me years to get over my mc)
Try alternative things if you can - massage, relaxation techniques etc
Try banning sex for periods of time so you can get close physically again. Few things can kill your sex life like infertility can.
My ds was conceived when we were ordered (by my masssage therapist) to stop trying for 3 months and to get close to each other again.
Try aromatherapy if you can - I hear so often that it's tension and stress that seems to be the enemy of fertility. Trying to get pg is the enemy of fertility somehow too isn't it? Too much pressure!
You can also think, like I do now, that if I hadn't had to wait this long long time for my dd then I would never have had her and she is the most preposterously adorable thing I've ever seen since my ds!
A philosophy which helps you accept things as they are means you can relax and let life happen at a pace that's right. Life is better when we don't try to arrange everything. Some people believe that things happen for a reason, I know it sounds glib and no one wishes us to suffer. BUT I know this was the road I had to go down to have these lovely children. I nearly gave up so many times. BUt then when we'd accepted we'd only have our adorable ds and I tried one last bit of fertility treatment (only clomid for 6 months) I got pg.
I also think that some of us just can't get pg til we've recovered deeply from our mcs.
Fertility is extremely mysterious. That's why you should not feel pessimisstic particularly as you are young and have another child!
Positive thinking, being kind to yourself, doing anything that reduces the pressure, grieving for your lost pg., cherishing your dd (as I'm sure you do).
It took a long time before I could accept pg news with a philosophical attitude. And I almost believe that I couldn't get pg until I'd really accepted that I probably wouldn't! One day we'll know more about the power of the mind over fertility. It is so painful a process.
Take care and all the best!