... Like a scene out of a bad rom com ...

Seriously man, I feel for you, but you shouldn't keep things like that, especially when you got back with her. Maybe you forgot about it though? Either way, I think if you just explain to her why you wrote it things will be ok. If she still doesn't want to be with you after you explain you're probably better off.

Either should have hidden it or thrown it away while u were bck togethet

I sense a second letter being written..

That sucks bro! Get rid of the evidence in the future.

Lighters are very effective in destroying things that should never be seen... I too use the write a letter method to work out feelings but I ALWAYS make sure to take a lighter to them or stick them somewhere I KNOW the person isn't going to find them. But in the long run, why would you want to keep something like that? The past is the past, work through the emotions and then move on!

convincing argument against the relationship? you can now try doing the opposite...

Count yourself as gone

"We were on a break"

You should have burned the letter when you guys got back together.