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You're actually good at writing, but the story is ruined by the forced and trite high concept. Far too much time spent explaining the inanities of this device, far too little in interaction between your characters. Do we really need to know that the device is like a phone, but also can send messages through time, and heres why it can do that, and blah fucking blah. Just mention (with one or two sentences maximum) that there is a device where people can send a single message to themselves back in time. Any more is pointless and in fact detrimental to the story.
And spend more time having your characters interact. You clearly have a talent for blocking and dialog. Focus on that.
And for fucks sake, the message needs to say, "Would you like to go out with me?". Then, the girl reads it and agrees.