Re: fear of ukemi
ummm.....sounds like you weight isn't be despersed over your shoulders properly. Your shoulders are taking much of the impact. This may be due to your weight. Your not getting over fast enough.
Really, I don't know because I can't see what is happening you take ukemi. I would be very careful, you may have already done some damage to your shoulders. Avoiding shoulder pain may also be causing you to do bad ukemi, causing new pain or injury. It wouldn't be a bad idea to see a doctor, and discontinue training.
Mentally, I don't think there is much you can do until the point you can due ukemi without pain. You being concerned and having a mental stop is something good. I wouldn't over ride that.