Re: landing on my back in forward roll
another thing you could do is when doing the rolls, you could also visualize yourself as a ball. if you think that you are rolling as a ball then there is no point of impact, everytime the ball rolls the weight of your body is evenly distributed so that you can roll freely or with ease. i myself try to picture out the wheel of a car that perfectly glides the street.
another thing to remember is that if you start in a right hanmi position while kneeling, after you completed your forward roll is you'll end up exactly the same position as if you are starting again, this is to remind you that while you roll you wont slam your lower back but instead put your feet under you to get up to the position as if you are just beginning again.
also start low or kneeling so that you are not farther from the ground to fear ukemi, once you've controlled that standing roll will be a breeze.
don't fear ukemi and take every bit of information you can gather to integrate or disregard in your Aikido practice.
happy practicing and enjoy your ukemi.