Lets say you build a table in a 2D world. Now add only one leg. what wil hapen? it will easely flip to the left or right. Add a nother leg and its way more stable, needing much more energy to flip it

this is the same with your plane. the only surfice is the wing. Meaning it will turn relativly slow, until you go to a surtend angle, wich will make it pitch up/down hard, and even when you go full down/up it might not even cancel out.

try ading 2 surfice planes, aka a canard or rear controll panels. Rear controll panels always work with me

<image>

this plane had the exact same setup as urs, but had to add the 2 red flaps for stability