I'm having a similar issue with a green morgan that I have been riding. I'd do your best to keep light, consistent rein contact. Keep your hands low and if possible try not to move your hands away from the saddle pad. My first inclination was to discourage my horse from leaping into a trot by asking for a few half halts because I too got unnerved. However, this just seems to confuse them and it really isn't fair to a green horse. Here's what I've been doing: I keep my lower leg away from the horse as much as I can, unless I'm giving an aid. Concentrate on using your seat and thighs to bolster you in the saddle, instead of clamping on with your lower legs (if this applies to you) Get a nice forward walk, and then gently bump your calf on his side and ask for a trot, but be careful not to keep your leg on. If he refuses to go, whip-wop the remaining rein on his withers and cluck. Keep it light-hearted. Praise him, pat him, most of the time they really just need to feel confident and be reassured. Try to match his tempo, and talk to him, this tends to help them turn their inner ear towards you, which really helps them focus and concentrate. Work on a bunch of transitions, work on a circle, keep the straight lines to a minimum. Good luck!