I had the same problem, and i solve it with focusing on one spot from the begging to the end of roll.
But the best way is just to practice them all the time because there is no advanced aikido without rolls/ukemi.
The Shodokan system is actually very gentle with respect to roles and ukemi.
First test - ushiro and yoko ukemi (no rolling at all).
Second test - forward roll from kneeling
Third test (about nine months into training) forward role from standing.
Most of our ukemi really is ushiro (without the roles).
So when I returned back to Canada I joined an Aikikai dojo that was role happy (in my view). I could do the roles fine but also got dizzy. Got over it though. Still don't think that you particularily need roles to do advanced Aikido - although you certainly need to be competent at certain other types of ukemi.