Is my water bottle okay to use?

I have this awesome water bottle that I’ve had for a year. I accidentally left it at school over break, with water in it, sitting on the window sill (so it got some sun).

I can’t imagine that would be too good for it, so I tried to wash it out, let it soak with soap overnight, etc.

This morning it was really hard to get all of the soap out of the top, and it felt slimy inside. There’s something dark, probably mold, inside the drinking part.

This is the bottle (although Wall-E, not Thomas), which is made out of #5 plastic.

I’d love to revive it, but I’m not sure if it’s possible. What do you think?