the turn around.

So here goes! My gf and i have been together for about 18 mo now but lately she seems to be standing me up. Something always comes up at last minute. We dont see e o that often so it bothers me because i know ill have to wait at least a week before i see her again. When she does stand me up i try not to act upset or be mean about it. But if i ask her regarding it she turns it around on me and gets upset. We talk alot and tx alot and we even have sexting. Shes tells me she misses me and she says she really wants to see me but idk why her words dont seem to match her actions. What should i do. Sometimes i just want to end this relationship but i really love her and dont do anything. What should i do? Is she loosing interest? Should i just not call her or answer her anymore or at least for a couple of day? Confused!!!