Dogmama "Tight turns - find an empty parking lot & do figure 8's. This gives you practice turning in both directions."
...and or practice in a park on grass? Soft landing pad.
check in with your body, you may not know how tense you are, some suggest "playing piano", see if you can wiggle your fingers. If they are tight RELAXE ;-)
Lower your shoulders, drop your elbows. I find I can put my body where it would be if I were relaxed faster than can think the tension away.
Oh, don't forget to breath.
On turns I keep my outside leg down ie: turning right, right foot is up. First you won't risk cliping the pavement with your pedal, more important you weight the outside of the bike, you will feel more stable, like carving a turn on skis ...at least I think that's what turning on skis feels like (I don't ski). Anywho... I don't know if this is right but I set up the angle and speed of a turn going in, drop the outside leg and then pedal out of it. Pedaling out of it really feels stable.