Well I dont think in that moment with Miles she wanted to kill herself. Im sure she was happy they finally got together.

I also dont think she planned to kill herself at all. But she was sad and upset. Her highs and lows were drastic. You could tell in her last look to Miles that she was zoned out and not even thinking about them in that moment. So maybe when she saw the truck in the road she saw that as her opportunity to "get out"...

However I would like to believe that she just fell asleep. I dont want to believe she killed herself. I know that alaska was depressed at times but I think the show made her seem a lot more carefree and optimistic than the books did.