I wonder if you might just need to build some G tolerance. Many of the manouvers are positive G but rolls do go from +ve to -ve and back to +ve and can catch you out. Try bracing your neck muscles.
anti biotics, alcohol, low blood sugar and dehidration also affect G sensitivity a lot.
When I first started I used to make myself woozie with 4G but now my tolerance has increased 6G is no problem.
I highly recomend ALan Cassidies book on aerobatics.
Perhaps we'll see you at a competition soon
see
http://www.aerobatics.org.uk for details. We welcome beginners at competitions (and the first entry is free!!)
Keep at it.. aeobatics is the most rewarding flying you'll experience. straight and level will never seem the same again.