There is a Lyric Feedback section where you could post this and it might get seen by more "lyricists" who could help out. Its tough to build off of since its for your ex and we don't know the small details, the small memories, of your relationship that could either pull at the heart strings, or jab her in the ribs if that's what you're going for.
Which direction did you want to take this? Its written for your ex, but are you trying to get her back... or are you rejoicing that she's gone?
Either way, I'd pick 3 or 4 memories that she would remember and write verses about them. In a good way or in a bad way, you need to make her "feel" the words.
I'd also word things so they appeal to more people than just the two of you, too. If its TOO specific, then you might get your point across to her, but you won't really connect with other listeners.
Good luck with this one!