A good idea is to get your horse into a frame at the walk and make sure you have good contact. Squeeze your horse gently into the trot (remeber leg into hand) and keep the same pressure in your hands throughout the transition. Frequent half halts (still leg into hand) helps too! Good luck!
I had this problem with a horse a lease way back and it was a bit hard to fix although not impossible. But I started walking him in tight circle then asked for the trot while he was in the circle and it wouldn't allow him to trot as fast. Every time he starts to speed up half halt which should be a bit for useful when he is in a small circle. Try to get a consistent trot if you can. Really work like this for a while then open up the circle little by little and eventually you should be able to use the entire arena. Hope this helped! (Let me know!) :)