I have a similiar situation I am dealing with myself - with the exception, that I did not get into a “relationship” with the girl, before things went south (or north, as we say here).
I too tried to show how much I care, how much I miss her, wrote her a haiku, sent her a rose and a teddy bear, gave her oregami flowers on several occasions. Yes, she was flattered at first, but eventually she became annoyed. It hurt, but I let her be… let her be for the time being, while working out the next battle plan. It was around that time I found this wonderful forum and community, and started experimenting with pheromones - have gotten wonderful results ever since - just that I don’t have such feelings for anybody else. I have, however become way more self confident in regards with women.
Thus far backing off has worked relatively well. While she is not knocking on my door, begging to have me, she still messages me occasionally, and is flirty around me - mostly due to the enhancing effects of the pheromones. I haven’t made any moves on her, though, as I am still doing my research to get the best mix for her. A-1 is likely the one I need, and I will make an order soon enough. When I get more concise signals from her, more chatting, then I will make the next move. But I have lived without her for more than a year now, and am 99% certain a few weeks or months more won’t make things any worse.
I know it may hurt, but Phertest and Mobocaster are correct. You may believe she is different, you had something special, and so on, and so on. But know what? I felt the same, and it did not serve me in any way. It only makes you go deeper and deeper into the Friendzone spiral of Hell. She may appreciate the attention - I mean which girl would not? But do it constantly, and eventually she will have enough of it. Your best bet is to wait and prepare for your move. I assume you are not stupid, you are a Northerner, afterall. 
Thus it should not be too difficult to sit down and think things through.
What might help you:
*I am not a fan of pickup artists and books, but there is one I found useful. “Magic Bullets” - it is available in the vast wilderness of the Internet, if you know how to search for it. - Not saying that you should not pay for it, but if you find it, and find it to be useful, then it would be polite to reward the author.
*As for pheromones: I would go for A-1 (Androstadienone) (you can get it from Liquid Alchemy Labs), IH, IS, P83 (in the 60mcg range), TAC, and perhaps P75(2,5mcg). I should stress out A-1, and P83 especially, as P83 is a good inhibitor (around 60mcg, as said). Then combine with what you have available to yourself and what you know to work best - either from your own experience, or go on some field tests and try out mixes that other people have tried and reported on.
*For the sake of it, you could even experiment with P95. I have gotten very interesting results with it - a lot of giggly girls.
Be aware, that I have no experience with A-1 myself. So feel free to do your own research - and do so on other girls (you should have no problem with that).
In conclusion, the worst thing you can do, is being in contact with this girl. Let her do the talking, if needs be, but don’t approach her, don’t be available for her, let her see you with other girls (with pheromones on - that should be obvious). Let any contact you may have be (seemingly) random, or out of necessity - i.e if you have legitimate business with her. Let her wonder about you, become more of a mystery to her. Perhaps you will be in a position to try again after 6 months. Could be sooner, could be later, but you need to distance yourself. No more 10 pages letters. Heck, not even any three word messages. Did I say no roses? No roses. Go on field tests, prepare for the battle, and win the war. Don’t prepare, and you will end up as dead (figuratively speaking) as all the other men you claim had no success with her.