I feel so sorry and sad for your loss;some insensitive people just don't get how devastating the loss of a beloved pet can be.I'm a cat lover myself,and when we got married,we got two kittens,sisters.One developed cancer aged 14---she had surgery,but the growth returned within weeks,and the vet rang to say that during surgery it was obvious that the cancer was very aggressive,and that my cat was already suffering.So I had to make that horrible journey.I spent a few minutes alone with her(she was unconscious),talking to her as I always had,and then I carried on doing the same while the terrible thing was done.Me and my husband wept for weeks,and her sister became distressed--she was mourning too.
My remaining cat is now 17,and has really come into her own(her sister had always been the dominant one)-----she is such good company(I'm very isolated),and is such a comfort;likes to be close to you,and always available for a cuddle.It's poignant though----she is pretty good physically,but obviously she's a great age,and inevitably,we'll have to deal with the loss of her eventually.
It's only in the past few months that we've been able to talk about the cat we lost,without crying.Animals are fantastic--uncomplicated affection givers,and great fun.
You will grieve at your own pace,but I hope it won't be long before you can think of Rusty without too much distress.It's a cliche,but time does heal,and to love always means living with the fear of loss.That's life,though it sucks.
You had to make a hard decision about Rusty---please believe that it was the right one,and that your love for him motivated you right up to the end---he is free from pain now,and he had a long and wonderful life with someone who looked after and loved him.If there's a Dog Heaven(I believe there is),he'll be free of pain,and having fun with the other pets you've lost.I always think of my cat as lying in the sunshine,and chasing butterflies.
I wish you peace.