I'm sorry at 13 y/o you've been hit with the ugly truth that life just isn't fair.
I'm also sorry that your parents have split. I never had to endure that, but many of my friends did when I was growing up and I know how badly it affected them emotionally.
I know things seem horrible, but I never even got to ride as a kid, much less get to ride once a week. My parents had 6 children and 1 salary on which to feed, clothe, and keep a roof over all our heads, so riding just wasn't in the cards.
Once you're an adult you can own a horse of your own, and although that seems impossibly far away, it really isn't. One of these days you're going to turn around and wonder where the last 30 years have gone!
Chin up. Yes, it stinks to have so much thrown at you when you're still so young, but you'll get through this. Adversity either kills us or makes us stronger, and I'm betting you'll use this as a life lesson.