I don't understand why you would continue to pursue her after she's ditched you more than once and cancelled your plans.

Adding to making you feel unimportant by doing that, she's let you know how far down on the list you are to her. That right there should be enough for you to walk away. While, yes, family and friends are important.. so is the person you've committed to a relationship with. If she has you dead last, you know where you stand, and you know it's unlikely to change.

Your best bet is to back off, hell-- end it completely, and find someone more worth your time that doesn't make you feel unimportant.