My friend is dealing with depression and I've decided I'm going to leave her.
Why would you leave your friend that's dealing with depression?
It's too hard for me to be around her when she's depressed all the time.
That is wrong to leave a depressed friend. She needs you.
That's too much of a burden for me to handle.
I know it's a struggle for you and if you can't be with her anymore, try to find her some professional help. Help her find a therapist that can help her deal with her depression. It's hurtful to abandon someone when they need you.