I see that this is your first time posting.
Stop by the Introduction section and say hello to everyone.
The link above did not give any specific links to your query - so I will add my 2 cents in here.
Caveat: I don't know what Im talking about, and I take no responsibility for anyone getting hurt for trying anything I say.
Seriously though, your not the only one who has an issue with Ushiro Ukemi. I don't like it at all.
There are different ways to approach this technique, and one may be better for you than the other - of course, best to talk it over with your 'qualified' instructor and go from their.
(Enough of the disclaimers...)
1) From a sitting position.
One leg in front and one leg kind of tucked under you to give you some 'spring' as you flip over.
2) From lying down flat on the ground. (I find this one hard, probably need a good set of stomach muscles.)
Put one hand straight over your head...(flat on the ground, not in the air)...and then pull your lower body up over you.
3) Sitting, with legs out, grab one side of your leg gi and pull up as you flip over.
How I do it:
Kind of a mix of 1 and 2...not saying its 'right', but its about the only way I can get myself to flip over. (Ushiro Ukemi is not what I consider a 'fun' past time.)
Hope this helps -