trouble with hapkido roll?

I am having difficulty with a hapkido front roll. I get how to do it... right leg foward, hands down, start rolling on outer edge of end and up the arm, feet flip over head, diagonal line from right shoulder to left hip, YES I GET IT. The thing is, I keep bending my elbow when I start the roll on my hand/forearm. I know I'm not supposed to extend it my elbow. But they tell me to put weight on my arm, which you kinda have to. But whenever I start to put weight down on it as I am about to roll, my elbow bends like a 90 degree angle. It seems like it just naturally bends like that when I try to start rolling. I just can't keep my arm straight and it's really frusturating sometimes. Can someone help??