Hi Jax,
Yep, the first thing family, friends said to me was Stay Positive, and yeah you try to, but yep its not happening to them, its happening to yourself. Its good to have people around who care, etc,
Do not feel bad about an animal not working out in your family environment, these things happen to anyone and you had the best intentions at the time so that is not being selfish at all.
I have myself been going through periods of anger, (more at the cancer)and not giving a damn. I think that is only natural and a part of dealing with having your life turned upside down in a heartbeat. Do not question yourself, just be. You have not changed, your just dealing with a crap card dealt, that is all. Stay strong to yourself, cry all you want, you are definitely not alone there.
Hugs.