This feeling is grief, and grief is a process with many stages and it needs some time. Do not force yourself, but do not write off the whole thing either. If you must, then take a break, allow your grief and emotions of disappointment to settle. Then, when you feel up to it again, come and just hunt some hunt marks in the pretty new zones, do some side quests (already found a whole number of rather fun ones), play the dungeons again without the burden of story. See how you feel after that.