I have read that Effexor is notorious for being difficult to come off of. Your best bet would be to taper off of the medication more gradually. Stopping this dose so abruptly is pretty much a guarantee you will have withdrawal symptoms. See your doctor and get him/her to prescribe you some 37.5 mg Effexor and take that for a couple of weeks, if not longer.
In my experience with Effexor, I had to taper over a period of about 6 months because I was having a lot of dizziness while I was coming off of the medication. My doctor was very good about tapering me off very slowly. (By the way, I was on Effexor for about a year and a 1/2 and my peak dose was 150 mg).
When I was tapering and got down to the 75 mg range of dose, I dropped it to 37.5 mg for several weeks. Then I took 1/2 of the 37.5 mg capsule to make it roughly 16 mg. Then took 1/2 of that to make it about 8 mg. Even when I tapered off that slowly, I still had some withdrawal, but it was much less severe than had I tried to do it more quickly.
I think you should probably keep taking the medication until you can see your doctor again. At the very least, break open the capsule and take 1/2 of its contents to give you a 37.5 mg dose and stay at that one for a little while. Your body knows it is not getting the chemicals it was used to having, which is why you are getting those terrible withdrawal symptoms.
I hope this helps!
You should always consult with your doctor before you stop taking your prescription. He/She will most likely want you to taper off the effexor slowly if you reach the decision to discontinue the effexor.