She may have realised that she in fact did not want to be with you, but you may have been intimate previously as she was undecided at that time. Some may not communicate this train of thought at all, which can be taken as completely inexplicable by the other party, unless a certain impression was given. Also, the attitude that "she's a type you will never see again" is not true. One will inevitably feel this way initially, perhaps it is due to the fact that a bond so rare has been broken, and one feels that recreating it may never happen. However, you will find that once you are free from the mental prison of loving somebody you can't have, that mentality disappears entirely.
Advice? I would say, make it known that you wish to know why she leaves - if there is a specific reason, you will most likely know, if not, and she doesn't really give a satisfactory answer, then it is likely that she no longer feels attracted to you - which seems possible given the description of the relationship. Some people just don't say "sorry I'm not attracted to you", or "I only like you as a friend". However, although you should not be too passive, DO NOT try to contact her whenever possible and cling onto her like you inevitably want to, otherwise it will push her further away.