I have a 3 year old who I am breaking in. Previous to me owning her she had two round pen rides as a 2 year old. She is super smart and very athletic for being a 3 year old. My only real problem with her that I feel like I can't handle is going in a straight line. I rode her for a good 3 or 4 rides in the round pen because that is what she was use to and had never seen a inside arena. As I started to ride her in the arena she would drift from the wall and try to cut the arena in half. I have ridden many green horses but never had a issue with not under standing how to go straight. To get her to go down the rail I have to have her counter bend and have my inside leg on to push her over. She is slowly picking it up but we are probably up there in ride 20 and I feel like I ride like a drunk beginner on her because of her inablity to go straight. I can walk her and canter her with her bent but I can not trot her the life of me and it is getting on my nerves. This not being able to travel down the rail was started to effect everything from riding a circle to just generally turning. Does anybody have any advice?

(When I say straight I don't mean perfectly straight I just want her to be able to walk down the rail and not be a good 20 feet away from the rail by the time we get to the other end.)

Baby horses often feel this way, and its a stage they will grow out of as they become stronger in their bodies and understand what is expected of them. Stick to the walk for now on straight lines, so you can continue to correct her and guide her at a pace that allows her to think. Slowly introduce the trot and canter as she "gets it".

This is also a situation where good ground work is key. Make sure she moves off pressure from your hand on her barrel that mimics the action of your leg. She should move straight over from this pressure, and with her whole body. The more she understands this, the easier it will be for her to move off your leg when you need it.

Counter bending can be a valuable exercise, but for now try to limit that. Focus on building up her ability to go straight, even if you need to correct her at every step and she can't truly hold herself there yet. It will come!

This is actually very common in green horses, and has less to do with "drifting" than it does to do with the round pen encouraging a horse to fall onto their inside shoulder. However, I wouldn't focus on this so much at her stage in training, and instead focus on steering instead of straightness.

What I like to do is what I call "point to point", which is where you just choose things to ride to. Cones set up in the arena, certain fence posts, etc., and use your legs, seat, reins, as you normally would to steer and go from each spot that way. It helps to have a goal, so that you do whatever you need to do to get from one spot to the other, and gradually she'll begin to understand the cues and you won't have to be as dramatic anymore.