You have every right to rant. Losing a parent is really tough, let alone having to deal with a diagnosis at the same time. As for other people, I think it is our society - they don't know what to say so they say nothing without realising how that hurts.
Isn't a side effect of tamoxifen mood swings? I've heard of a few people really struggle taking it. But give yourself some credit - you've been through two major life changing events. There will be an end to it and it looks like you've got the support of a beautiful little girl. My girls are 15 and 12 and gosh i hope they don't have to go through it.
Be proud of yourself and you rant any time you want *^_^*