Hey there! Your problem is, your CoG (Center of Gravity) is too close to your CoL (Center of Lift). Your CoG should always be well forward of your CoL. The distance ahead determines your flight stability. If it is just in front, like yours currently is, it will have a tendency to flip, because the nose does not drop in a stall to increase airspeed and unstall the airplane. Now, a CoG that is far forward of the CoL will be extremely stable, sometimes too stable. It will resist changes in pitch. This can also cause your plane to ground loop in a tricycle gear configuration with a nose wheel. So, generally just a few feet in between the two is great.

As for your airplane, you can either move the CoL aft by moving the wings back, or you can add dead weight in the nose. I suggest the first one. To see the CoG, CoL and CoT (Thrust) press the spacebar.

I know that is a long explanation, but I hope it helps! Good Luck!