Without wanting to become too... detailed, can your... well, "size" in the lower department be connected to that issue? I have read on multiple occasions that "larger" guys are more prone to erectile dysfunction because getting proper bloodflow is harder (no pun intended) to achieve. I am personally rather large and apart from my morning wood (which is, when I have it, hard enough to plow through a frozen acre), my erections, which I don't get many of to begin with, are usually rather weak and arousal doesn't last long, I've heard that there's certain exercises that might help (kegels, for one), but I never bothered trying anything in that regard since... well, my equipment remains unused, so I have no motivation to do that.