Metoprolol Problems

I had a mild MI 8 years ago. At that time my BP was, and still is, moderately high. 135 - 150 over 75 - 90. My doctor prescribed Metoprolol as well as several other "maintenance" drugs including Lisinopril and Plavix. After a couple of years on the Lisinopril and Plavix, I was taken off of those medications by my cardiologist and left with a maintenance regime of Metroprolol and Simvastatin. I am not trying to "wean" myself from the Metroprolol but, if I miss a dose, even for several hours, we are not talking days here, I experience severe heart palpitations. These are combined with shortness of breath and panic/anxiety type symptoms. Is there a reason, other than physical dependence, for these symptoms after missing even a single dose??