Your post struck a chord with me, too. My DD will be turning 2 next week. And while these past two years have been the most wonderful of my life, I also feel a sense of incompleteness. I had a miscarriage last June -- so I went through what you're going through now back in January when I would've delivered. I think it's perfectly normal to mourn a milestone that never came to be. I also feel like every passing milestone in my DD's life, as joyous as those milestones are, is a reminder that she's that much older and that I've gone that much longer without being able to conceive a sibling.
I think you vented at the right place -- who else would understand better than those of us dealing with secondary IF? Hugs to you.