Re: Problems with Ukemi
I haven't been doing aikido for very long, but i've thought
quite a bit about ukemi. One general thing to consider is that you may be pushing yourself too hard for your level of experience. We have a first kyu in our dojo who is similar to you in physical build.
His technique is excellent, but he still takes our ukemi class
for beginners so he can focus on just making his contact
with the mat more safe and graceful.
My general advice would be to slow down and treat your ukemi
as a stretching exercise. Let all your joints stretch smoothly and slowly in a relaxed manner as you lower your center to the mat. Pay attention to your posture as you set yourself up for a forward or backward roll. Then complete your ukemi by letting your own body weight naturally take you to the mat (i.e. don't "throw" yourself.)
Obviously, you should have a cooperative nage when you practice like this...
Just my quick thoughts on the subject. As Jun said, more specifics would help others help you.