long term effects of Effexor Er

I have been on Effexor Er for 12 years and am currently taking 75 mg one time a day. I am 53 and started my first migraine on May 11, 2008 and it has not gone away. The doctors don't seem to know why migraines started for me at this age with no prior history. They are giving me depakote to make the pain tolerable. After reading the letters about Effexor withdrawal, I wonder if my constant headache could be from taking the med for so long. I am not trying to withdrawal at this time. Has anybody experienced the migraine before withdrawal? And how long do the withdrawal symptoms last? I would love to hear someone's success story of withdrawal!