For a plane that you didn't quite know what you're doing. Its okay. ther are a number of improvemnts you could make however which will apply to every build you ever make.
I'll upload an improved vesrion tommorow (when the limit for daily uploads resets) which will still look a lot like this but be built more properly.
but for now, key thing you did wrong is added dead weight. you tried to balance the plance to your center of lift. you should be balancing your wings for your center of mass. this means removing all dead weight and fuel. putting the centero f lift a bit behind mass. then filling it up with fuel with out moving mass too far from where it is normally.

secondly is wing loading. you just need to make your wings bigger. not wider, you don't need more wingspan. I just mean bigger wings which connect to hte plane further on its lenght. this goes for all wings. but don't go too extreme.
you can also always check your wing load by opening the menu and hittin the information button. (that i in the circle). wing loading is put simply. how much weight your wings hold. bigger wings have lower wing load. this is good. means you turn faster and fly more stable.