Yeah, I think your taper was too fast. Like you, I was on Effexor (venlafaxine) for a long time—18 years. I warn you, Effexor gets stored in fatty tissue all over your body I was only ever on 25 mg Effexor/venlafaxine (for hot flashes caused by breast cancer treatment); I didn't develop withdrawal symptoms until 6.5 weeks AFTER I slowly tapered off. I think it took those weeks for all my body stores to be used up.

I don't know if reinstating would help you, or not. You could try it for a couple of days and see. In my case, I went into the whole process of getting off Effexor without realizing how difficult it would be—my oncologist just said to cut my pills, take a quarter less for a couple of weeks and cut again by a quarter until I was off. I had what was left of a six-month supply and no prescription refill. I didn't know I was in trouble until 6.5 weeks after my last dose. I started researching Effexor withdrawal BECAUSE I was experiencing it. Once I read others' stories, I didn't think my reinstating after so long would do any good. What did help was to take a number of OTC supplements—GABA (a brain neurotransmitter) and ESPECIALLY, L-tryptophan, an amino acid which the body changes into serotonin. (NOTE to others reading this post who are still on Effexor—Do NOT take L-tryptophan, or 5-HTP while still on Effexor as this can trigger serotonin syndrome.) You can click on my name and go to Discussions to read my previous posts and see how much/how often I took the various supplements.

I'm sorry the Prozac "bridge" didn't work for you. Did your doctor cross-titrate between the two antidepressants (have you go down on the Effexor and go up on the Prozac), or did he just have you switch over to Prozac?—
(https://www.depressionforums.org/forums/topic/107532-coming-off-of-effexor-with-prozac-bridge/)

A commenter to the above forum wrote, "If you have issues with anxiety/irritability/etc., then you can take a benzo temporarily, like Xanax, Ativan (short-acting), Klonopin, Valium (long-acting), etc."
I can attest to the effectiveness of Valium for those issues. As well as my daily regimen of OTC supplements, my oncologist prescribed 5 mg Valium/diazepam to take as needed when my akathisia and/or anxiety got too bad. (I took those Valium sparingly, seldom more than a 1/2 tablet for a day, or two, as the prescription was for a small quantity of pills.) A couple of months later, I ended up going to Care Now after a particularly stressful Friday at work because my agitation was through the roof. The doctor I saw was familiar (very unusual!) with Effexor withdrawal. She had me stay home for three days and gave me prescriptions for l-methylfolate (an easily metabolized version of Vitamin B) and Valium. I was to take a whole Valium twice a day and rest. Her thinking was that I needed to use a significant amount of Valium to stabilize (she described it as "getting ahead of the anxiety") and then back off the drug.

Hope this helps.