I'd go back to basics and do a refresher course for her. Walk/trot her inhand clearly saying 'WALK ON' 'TROT' and 'HALT' or 'STAND'. You can use 'aaaand' before a command too, as this will teach her that 'aaand' means a command is comming and she needs to be alert, using 'aaaand, STAND' is really useful, and is what my youngster took to best, but every horse is different :) If she doesnt halt on first command, give her a slight backwards pull on your leadrope towards her chest while repeating the command again to reinforce what you are asking of her.  Once you can slow her and halt her in hand without any trouble and on a loose leadrope, then lunge her. This will get her used to doing it independantly. If she stops listening, walk her around the lunge circle inhand again until she is listening and doing as you wanted, or alternatively, is she isnt scared on the lunge whip, place it in front of her at the shoulder to reinforce the stop signal. Remember to give praise when shes does it well, though I prefer not to give them treats as a reward as then they can get pushy and keep asking. Once she is lunging well and stopping/starting/slowing pace on the lunge, you can do a bit of loose schooling in a slighlty larger area, but have your leadrope/whip handy if you need a light reinforcement. You could then go onto long-reining her, which will teach her in her riding equipement and in a similar reing position to stop when asked. You could also tack her up fully on the previous steps so she associates the commands with her riding time. You may be able to skip some things.do things differently, but its entirely depended on the horse and how fast she learns. Once she is doing well, you can ride her starting in walk and make sure she is stopping before you move to faster paces. It would also help to have someone on the ground to reinforce the command is she wont stop and may have to be told from the ground. Hope this helps! Ex-polo ponies are normally on the go quite a lot of the time, and I find that actually using your seat more to stop them and having a slightly looser contact can help. Quite a few polo ponies relate a strong contact with a faster pace, and also having a strong contact can make you more tense, so using your body more could help alot :)