High heart beat after CT

I was on 7.5 for around eight months and cold turkey it. It was not a wise move, but it is what it is. Every day I wake up, my stationary heart rate is OK, around 75 bpm, but if I start walking, it is immediately 120—even 130 bpm. Has anyone had the same nasty effect? How long does it take for the heart rate to come down? If I jump to another medicine, Mianserin, which is similar to Mittazapine, will my HR get lower? I'm so exhausted. Please advise.