I didn't have this much of a problem but my right arm has always had more endurance and strength than my left. Some suggestions i've heard some of my buddies do is start with your weak arm first when working out, even if you rotate dumbell curls, you should start with your weak arm and rotate, once you get finished with your weak arm but your strong arm can do more, stop. Over time your weak arm will develop and your strong arm will still be able to keep up, well because it's your strong arm. Also some things they suggested are doing natural everyday things to help increase your weak arm, like carrying a heavy bag of groceries with it, using it to carry your schoolbag with all your books, just trying to incorporate the weak arm more into your daily life will help strengthen it.
That's what some of my buddies at the Gym suggest, my arms have kind of evened out but my right arm can do a few more reps (Say 2-3 more) Of course this depends on what i've done/eaten that day too, for Thursdays( My arm day) If i've eaten like crap i'll feel like crap and I may not be able to do my dumbell curls correctly, or if i'm tired from work I may not be able to go as heavy as I usually do. Having one arm half as strong as the other? Did you injure yourself

or has it always been that way? Something more is to make sure you're working out your legs as well, your body will want to maintain equilibirum, if your lower body gets stronger your upper body will get stronger, any big guy at the gym will tell you that.