Do you suck at Romanian deadlifts? Does your lower back hurt? The two are related. And fixing this problem is simple: do the single-leg RDL.
My preferred setup for improving the single-leg RDL involves loading the pattern with a dumbbell in the hand opposite of the leg that's in contact with the ground. This "contralateral" loading works the anti-rotation plane of resistance during the hinge, which helps to enhance lateral hip stability to an even greater degree.
Single-Arm Single-Leg Romanian Deadlift with Iso-Hold
While this movement can be loaded on the same side or with two dumbbells (or barbell for that matter), the single dumbbell in the opposite-side hand will help you get the most out of it.
You won't master this movement without stability. So tense your glutes, core and shoulders before you start the lowering phase. If you're having balance issues, focus on bracing yourself the same way you would during big barbell lifts.
Move slowly through the eccentric (lowering) range and lead with the hips. You'll get a hamstring stretch at the bottom, so keep the spine in a neutral position to avoid losing posterior chain tension and to avoid rounding over. From there, drive up hard and squeeze in the top of the movement with a big flex of the glutes, adductors, and core.
Make It Tougher
If you really want to challenge yourself, on the last rep of a set do a loaded isometric stretch at the bottom of the range of motion. Just hold the bottom position with your hamstrings lengthened for as long as you can while maintaining balance and keeping an active contraction in the hams, core and glutes. This will be tough.
Since these require high amounts of motor control and stability, keep your reps between 5-8. It'll also help you avoid making this a balancing act. Touch down slightly with your toe between reps and keep proper tension and stability. If you can do this, you'll see your loads linearly progress, and chances are your back and hip strength will start boosting your big lifts as well.