Ukemi is a function of conforming and acceptance. Fear creates tensions, which result in injuries.
I start my students off on the ground to learn how to move softly on the ground. I then work on how people can relax in order to move closer to the ground. Think of the forward roll as becoming a tire. You soften your entire spine so that your spine becomes circular. You allow your head to rest where the axle of a wheel would be. Feel yourself extend soft energy and hold that soft circular form. If you allow your hips to soften backwards, you will begin to fall forwards. Hold your form and allow the momentum to guide you into a soft, circular roll.
Go slow, if no momentum is provided by tori let gravity pull you over when you begin to fall, don't throw yourself into the ukemi.