nobody will ever feel the way I do about her and I told her this.
I know how hard it is (recently had my fiancee do something similar), but the above just isn't true, and you cannot make it true.
People change and grow, sometimes in different directions.
To quote my ex, which I think is a somewhat valid analogy, is that everyone has a contract, an almost predetermined amount of time and impact they will have on someone's life.
If it was meant to be, it would be, but as it is not, better to celebrate waht you had, experienced and learnt, rather than mourn what you have lost.
You might meet someone even better in the future. In fact, you probably WILL, as you will change, have changed, and someone better suited to you will come along as a result.
Chin up mate, things only get better from here.
But the staying friends thing, difficult, I've never personally managed it, it's just too awkward most of the time, especially when trying to have a conversation with a person that you have spent most of your time with as your partner.