Vipassana meditation comes naturally; Samatha meditation incredibly hard.

Let me start off by saying that I can do Vipassana meditation for about 20 minutes at present. Accepting thoughts as they arise, noticing sensations without judging is relatively easy, compared to doing Samatha meditation. However, even though my body may feel relaxed and calm after doing Vipassana, my mind is still cloudy and obscured (I suffer from brain-fog and Depersonalization Disorder). Depersonalization Disorder is a sense that you don't really exist, almost as if you are outside your body, lost in a cloud. I feel calm, but my mind still feels foggy. If I focus on the breath (Samatha), I find that I only manage to sit still for about 3-4 minutes before becoming physically agitated (to the point where my body will curl up in tension). I feel as if I need to go for a run; my mind kind of shouts at me, saying "Go, go go! Get up. Go do something". There is no anxiety, just physical and mental agitation. My mind feels fine doing Vipassana, jumping from moment to moment, letting things be. But when I tell the mind to focus single-pointedly, all hell breaks lose. Should I carry on with the breath, even though I can only do it minutes at a time, or should I do something entirely different?