Hopefully your horse is generally more responsive to your aids when you ask him to go forwards! If he isn then spend time improving this aspect of your riding√¨ alternatively you might want to start improving your horses responsiveness to the aids on the lunge first.
Once he is going forwards properly when asked, introduce some leg yielding work, initially in walk so you can both get the feel of the movement before moving on to trot. When he is happy leg yielding from the three-quarter line to the outside track, and vice versa, try introducing leg yield along the short sides of the school. You should be able to encourage your horse into the corners ‚Äödont go too deep or you will make the exit from the corner too difficult.