Re: fear of ukemi
Weight is definitely a factor. When I'm lighter, my UKEMI is far less stressful.
For forward rolls, a lot of people sort of leap over their extended arm when they "roll". Are you doing this?
I suggest, as above, beginning on one knee with the other leg extended behind. Put the arm on the same side as the forward knee down, elbo about even with knee, hand back toward groin. As you roll, concentrate on feeling every moment of contact from the tip of your little finger to your hand blade to the side of your arm all the way down.
Then concentrate the same way when you try standing. If you don't feel the mat on your finger and arm, your probably leaping onto your shoulder.
You can feel self-conscious doing this during training as you may feel as if you're slowing down NAGE. Go ahead and slow him/her down. You're both practicing aikido, not just the person throwing. This is important and can/will prevent injury.