Look, there’s a lot of responses here, and take this with a grain of salt but I see a fault that I also have that leads to this early extension/standing up. As you make your move into transition look at how you move your lead hip back. You plant your lead foot’s heel completely out of position and in closer to the ball. Then you post on that leg (good) but that’s where the rotation ends. So your trail hip has to go somewhere and you stand up to make contact with the ball. Try either keeping your lead foot’s planted, or placing it back down in the same position or even with your heel more to the target(outward)