Circles are your best friend, lots of inside leg
Tighten Outside rein and outside legs must be on. Inside rein should be loos
Hi Maddy...encouraging your horse to flex to the inside is very complex without seeing you on your horse, but you can try circling your horse to the inside each time you see their nose go out. You can ask your horse for a shoulder in on the next corner. You can also try lifting your inside rein gently when you notice your horse is flexing out.