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You have a purpose in life, which is why you're here. Killing yourself will not solve anything. And besides, you'll just keep coming back to this world until you have fulfilled your purpose anyhow, so there's no running from it.
On another note, just give up on her already. You say it's impossible to stop thinking about her, but nothing's impossible. Find a hobby/job you like, a specific book genre to read, a girlfriend/boyfriend irl, anything to keep your mind set on something else. If you keep trying for her like this, there's no way she'll ever come back to you.