What is mourning?

Mourning is such a dark word to live in the joyful world in which we live with our sweet Fiona.  Why does this word still exist in my vocabulary as we move through our lives together.  I have said before, feeling sad does not mean that you are not grateful for the things that you have, and I think mourning something does not mean you don't love, celebrate, give thanks for, or cherish.
Last month I was feeling this vague "funkiness" for lack of a better word.  For many days, I couldn't really put my finger on it...why was I feeling off most days, why was I feeling sad sometimes, and why was I feeling like shutting out the rest of the world.  Then it occurred to me...I was mourning something. 
This may sound strange to some, so bear with me while I explain.  Last month, two very good girlfriends gave birth to beautiful, healthy babies.  What a joyous occasion!  I am genuinely happy for both of them and was so happy to see pictures of their little ones snug as a bug at home, wrapped in soft baby blankets, cuddled with their puppies or toys...that is truly an example of the joy that makes life worth living.  And I didn't notice at first...or at least I didn't make the connection...that these blessed, joyous events were igniting a (strange to me at the time) period of mourning in my heart.  I realized that seeing others bring their healthy babies home just days after welcoming them into the world, also made me feel terrible.  Not because they don't deserve it or because I wanted their babies to stay in the hospital...never!  How lucky are they that they have healthy children and no child EVER deserves to be sick. 
But it made my heart ache for an experience that I was never able to have.  I never got to bring my newborn home and wrap her in soft blankets and cuddle her up with her puppies and spend hours rocking and nursing, losing sleep over hungry tears.  Funny thing is, Fiona is happy and healthy, growing, developing and thriving now at home.  I am back to work and feeling more whole again.  I thought I was okay now, that I was done mourning Fiona's diagnosis or Fiona's medical challenges.  I thought that life had moved on and that I was normal again. 
At first I thought that mourning this loss was a sign of weakness, or that I wasn't grateful for the health Fiona has now.  I thought that admitting I still had something to mourn meant that I didn't cherish the baby that I DO have.  And thank God for an amazing coworker that, through sharing her own experiences, has helped me realize that mourning will always be a part of our lives and that I need to realize that doesn't mean I'm a bad parent, ungrateful, unhappy, or disappointed with my child.  It is just a fact of life.  As Fiona grows and as others' lives move forward, there will always be little things that may hit a raw spot in my heart and I will mourn the loss of some thing, dream, experience.  And I know that I am not the only person that experiences this and that the idea of "being okay" and what that means may frequently change for me. 
What is mourning?  Its part of being human and being a parent of a child with different abilities and medical challenges.  It will always be a part of my journey and that is okay.