For the Cuban-8, as well as many of the other basic maneuvers, you need to learn to do them "by-the-numbers," 1 control at a time.

Start the Cuban-8 flying down wind, full power, parallel to your runway and a safe altitude high.

Don't do a full 3/4 loop at first. Instead of 45 degrees down angle, shoot for 20-30 degrees at first. Start the roll just as the plane gets the nose down because on a trainer, the nose is going to drop a bunch so you need to start very shallow. Doing this should allow you to finish at 20-30 degrees nose down. This shallow angle will keep the speed from building up so much.

Release the elevator and put your thumb or finger beside the stick and push it over. This should help you to not hold in any elevator. You will probably want full aileron.

You are gripping the stick too hard so you can't feel the different inputs. You need to go up-release, roll-release, up-release, roll-release, up to level. Practice releasing, or very nearly releasing, the stick so it can spring back to center. Check with your hobby shop and see if they can install stiffer transmitter springs.

It is actually much harder doing acro with a trainer than with an acro plane. You need something at least like a Stick, but built with a flat wing, an Ultra Stick or a Big Stick with the wing flat. These will roll a lot better.