Sorry to hear about Shell, Neill. It doesn't matter what your pet is, it's that you had so much of yourself invested in her. I think the best thing to do is wait and morn your loss. But when you're ready, you should get a new pet and start a whole new batch of memories. As you said, "I could have spared myself what I'm feeling now by not ever having brought Shell home but that would have meant I missed out on the 12/13 years of joy she did bring me."
You know you'll really never get over the loss but you will find joy in sharing life with another pet eventually.
Again, sorry about the news and hang in there...
