Maybe yes, maybe no. The issue could be metoprolol, in that it is keeping to heavy a lid on your HR when you need it higher, such as when you want to go for a walk. However, there could also be cardiomyopathy at play here, maybe valvular problems. Have you had a recent assessment of your ejection fraction? If you have enlarged ventricles, or thickened walls, scarring in their substrate or collagen deposits, or if you have stenosis or regurgitation the wrong way through one or more valves, these can all rob you of efficient pumping of sufficient volume to meet your needs. Sound to me like it's time for a new brain to take a cold and sober look at your heart function and structure for adverse morphology.