Increase the iso and stop down. More DoF makes focus accuracy less critical, and the rule of thumb for not getting camera shake blur in the shot is to shoot at a shutter speed of 1/
focal_length or faster. In this case, a 50mm lens, that means 1/50s or faster. And this is assuming you know
how to hold your camera. You have a dSLR. You can use iso 800 without fear. Honest and truly.
The sharpness sweet spot on the 50/1.8 II, btw, is around f/4-f/5.6.
Also, with focusing, sometimes it helps to turn off all but one of the AF points, so that you can tell the camera exactly what it is you want in focus, rather than letting the camera guess. When the apertures get wide and the DoF gets thin, you need to be able to place your focus much more accurately. Aiming for high-contrast targets (ideally, where black-meets-white) will help. In low-light the 50/1.8 can get wonky in AF accuracy because there might not be enough contrast.