i have an ex race horse and she does the same thing... haha
I have had her over a year now and have been working on this the whole time. She is soooooo much better but it just takes time to get it. Are you using a line or a round pen? I have found that I have to be VERY firm with her. I think at the track they just let them go till they want to stop and then they are done.
On the line I have found that I have to get behind her shoulder and make her go forward. Once I get a circle or two she usually settles in.
In a round pen I just have to stay out close to her and again make her go forward. It's like you have to support them every step of the way.
Also I switch up witch way I start on, one day starting on the left and the next on the right ect.
Good luck and let me know if you have any other questions!