Watch the way an experienced pilot moves the sticks for smooth laps around the field. It takes VERY small stick movements to fly the model.

Another place that may make it seem unstable to a beginner (even if properly trimmed) is you may be holding the stick off center too long,,, then when the plane continues to bank further, you input MORE in the opposing direction and soon end up slamming the stick from side to side. A way to see that you don't need to hold the stick over (but a bad habit to get into...) is the bump and watch method of flying the model. Bump the stick and let go... watch the result. Bump to make any correction... and watch the result. Its a sloppy flying technique... but it helps prevent overcontrolling. If the airplane is trimmed for level flight and you bump the stick... it will give a visible response.

Properly trimmed (with just a few clicks of rudder trim from straight and level) a trainer will fly lazy circles as long as you let it. The more gentle you are with the controls... the less trouble you will have.