I know this won't be what you want to hear, and I'm sorry, but it needs to be said.
The sooner you accept that she's not yours, the sooner you'll get over what you're feeling. It's not the girl's fault for being at the wrong place at the wrong time and it's not her fault you feel envious over her either. It's normal to feel a little sad or heartbroken if things don't go how you would have wanted, but this is a bit of a step beyond that. I'm sorry, Roxas, I know that it's challenging dealing with our less than ideal emotional processes, but you cannot point at a girl and say she's causing your depression if she isn't doing anything directly. Please, please understand.