I recently purchased an ex-racehorse and am re-schooling her as a general riding horse.
She is going quite nicely on the flat and jumping small jumps. She hacks out in company and is great in traffic. However, I cannot hack her alone as she refuses to leave the other horses. I realise in her former life she would always have been ridden in a group, and wondered if anyone had any strategies to help me with this problem.

I had an ex racehorse at one time and compliment you on owning a lovely animal as they are. I never had this problem with him though and suggest maybe you will have to take the time to lead him out instead of riding him out until he gets the idea. Treat him on the way but not on the way back.

Is she scared to go out by herself?
Or is she just been stubborn?
If she is scared take out a little bit then next time go a bit further and so on let her know everything is OK, build her Confidence up.
If she is been stubborn you are going to have to stand up to her not belt her around or anything like that, just let her know who is boss and don't let her get a way with because if she gets away with it once she will think she has won and she will get worse.
I hope this helps