Yes I agree with the others. I remember going out hacks on school horses and they knew exactly the spots where they were going to gallop or cantor and it can become dangerous. She will feel your tension and think something is about to happen. I had to keep giving my own  mare unexpected halts. Do this a lot when out. Make her/him halt, stand and gently into walk.  Use your voice--(WHAT A GOOD GIRL) Then same in trot. Keep everything calm.  This worked with mine as at first I couldn't get her to stop.  Be firm but calm. You'r in charge. Hope this helps. I know how you feel!

maybe try hacking her with other horses, and do u gallop her alot on hacks? if u do try doing a whole hack walk or trotting and dont canter or gallop, because if u go on hacks on ur own and u gallop her at the exact same area as u always do she will get cocky and excited like u say and sometimes it can lead to tanking off because they know what they usually do and where they go, i hope this helps it did on mine and my sisters horses