Well I'm no expert but here is how I get my horse to bend.
First, move his body first, as bending should be with horses body, not just the neck/head. Push his body with your inside leg, and gently "sponge" your inside rein to get his whole neck and head turned in, not just his face. A good excercise that I use is to make a circle in each corner (not each end, just the corner) with only your legs. Keep your inside shoulder and hip up, turn your head and body towards the center of the circle (but look up!) and push
him into a round circle. If his neck is out you can squeeze your inside rein, but mostly leg.
Also try circling around things such as standards, and work on serpentines. When I was working hard on it, I was able to have my horse do tight, rounded circles with just my body and legs at the walk, trot and canter. If you work hard enough, you will be great at them, it just takes practice!