Help convincing my mom to get a dog + a bit of drama :/?

My mom hates animals, because of the up-keep + finances (we're not low-income at all though). However, I take care of our current animals (kitty + tarantula). I LOVE animals. I need them really. I'm going through one of the most difficult periods of my life. I have severe OCD. I literally cannot function in the real world, where stress is involved (i.e. school and work). Applying for disability, until my condition is at the level where I am able to deal with it, then I will get off. I have to live with my mother, because of it. I am getting therapy and my medication has just been adjusted. My sis recommended volunteering for the SPCA, but that is not enough. Because when you are alone at night, you just need something to cuddle, y'know? Tarantulas don't cuddle, lol. My kitty has a 90% chance of having malignant cancer. I am having a vet. appt with him this week. Meh.. I do not want a cat, because it will remind me of him, and there is no replacement. He is a great kitty. My sister just interrupted me, so I completely forgot what I was going to say next, but anyway..I need advice on talking to my mom about this. She is not a sympathetic person at all. So, yes. Encouragement and advice, please?