I can relate to your feelings as I experienced that heavy heart and also a lot of guilt over wanting another child when my house was already full. As Cotton said, 'the heart wants what the heart wants.'...why is that so wrong?
Just because you have already been blessed with a child, why should it be wrong to want another child and feel so sad and depressed when that child hasn't made it to earth yet.
It's part of the process though...all these feelings and it's good to feel them, experience them, grieve the fact that it has taken years to get to your dream, years of being in the depth of your soul all this time...but trust that you will move beyond this place.
Although I didn't work with a therapist when we were ttc, my gut told me that I needed to stay in touch with my spirit and so I started practising hatha yoga. Learning to relax in my yin poses, some of them painful at first taught me to relax in the pain of my ttc journey...to keep breathing and to let all the good of the moment wash over me as well...feel the pain and trust that I would become more balanced and flexible.
I would also get up very early on many mornings, tuck my yoga mat under my arm and climb the mountain behind our house, I would watch the sun rise and marvel in all the beauty in the world. I would feel connected to it all and if I were a part of all that beauty and magic, I felt I could manifest ANYthing that I dreamed....and so I would sit and dream about the baby I felt was missing in my life...I would see that child in our life....feel the joy of holding that child and raising that baby...and then I would meditate.
I don't know how I got on the spiritual path...I think it was watching Oprah...and she had Gary Zucav on her show one day...one of the first books I read was Gary Zucav's 'Seat of the Soul' and then another spiritual teacher she had on was Elhert Tolle...oh and it continued, Dr. Deepak Chopra...Then I started to listen to Dr. Wayne Dyers CD's as I drove our children around...cd's like, 'Power of intention'....'how to manifest your dreams'....'meditations for manifesting'...etc...also I learned the power of positive affirmations and I used to say all the time, 'I am strong, healthy and fertile'...I still say this affirmation...even if our last 3 children have come as a result of using de's...I'm still fertile...I'm abundantly fertile in creating a happy life for my family...we are all creators.
I don't know if any of this will help you...each of us has to find our own path but if you stay in touch with your heart...feel the pain, but start trying to find a way to heal your heart as well...yoga, meditation, visualizing, positive affirmations, learning to love yourself, heal your heart, forgive along the path and then trust and let go.
I know for me there were times that I just wanted all the pain to go away...there were times that I got off the ttc path and just tried to live...but I kept hearing that whisper and my soulbaby would come to me in my dreams so I had to continue. If you shift your perspective to appreciate the journey you have been on...to look at it as a gift, to believe that it is teaching you to be the mother you need to be for your next child then it gets easier.
Until you are beyond this...just keep breathing...I really like something I just read recently that sometimes in life all we have to do is just show up...just show up...let go and trust...find a mantra that will keep you putting one foot infront of the other and in time you can look back as well and give back to those who are coming behind. IT WILL HAPPEN and when the time is ready...your soulbaby will come. This you must believe...until then, may you be well, happy and peaceful.