I can only talk from experience when I say how I have too went through some issues, both with mild depression which was believed to have 'come on' via having anxiety attack issues. More to the point when I had my little dog, who sadly passed in April, she too was my rock. I know exactly what you mean when you say how she was your constant companion. Even something as simple as getting some toast, you know she'd either be sitting near you or hogging your warm sofa seat waiting for your to return. My dog too slept in with me, every single night and I long to feel that 'pring' feeling of her paw patting my stomach - walking but lying still-, like she always did when she dreamt.
My dogs things are all over the house and I honestly say this with the kindest heart - don't throw them away. I had my little ones bed in the livingroom for a good 3 or 4 weeks before I put it into the spare room. Her water bowls lay on the floor for a week, until I realised it was just dirty water and wouldn't be doing anyone, not even her, any favours! Her toys stay around and always will, I also have her little coat that sits near my chair in the livingroom - it's behind me so not a constant reminder but a nice feeling when I see it.
I too agree about those who have never experienced the love of an animal, I don't think you've properly lived until you do. The end result is obviously extremely painful, but I know that's a pain for me and not what she is going through. So I know if someone told me I could rewind time I would do it all again, in a heartbeat, as long as it was promised it was me feeling the pain and sadness like just now and not her.
Take great comfort in the fact that you were 'it' for her. You know when people say things like their partners or their kids are 'it' and that's all they'll every need? That's you. She lived the life she did because you got her through each day.
Animals are like humans in the sense of feelings but we all know how if they want to go lie by themselves, then they will! Where as a human might think "hmm, maybe I'll stay and not walk incase they are offended." She chose to always follow, sleep in with and cuddle up with you. You're it :)
Keep your head high Christine, she's happy today and is still around because you gave her all that love! Take your time to go through it and never feel guilty about it.