Hey there,
Although I have never been on Seroquel, I am currently withdrawing from Effexor. I have all the same symptoms you described. The sheer panic attacks out of nowhere, very fast and sudden heartbeat, diarrhea (tmi), and horrible fatigue. My psychiatrist told me that I am returning to my original condition as well, but my anxiety was never this bad before! I have read similar stories to ours, and I genuinely believe that these are all withdrawal symptoms. The “returning anxiety” excuse is not good enough for me because how are we all having the same exact symptoms? I have never had such intense panic attacks and my emotions are all over the place as well.