Hey I ride a fifteen yr. old standard bred. He raced when he was a colt. Now he is absolutely fantastic except for while trotting especially away from a jump or if he spooks, he'll start pacing which is extremely hard to sit to and post to, so either I have to speed him up or slow him down... what can I do about this

Maybe he can be getting excited about his past an he wants to race? Set up a small jumping obstical to jump and wirk with him once he doesn't soppk with that one move onto another one

Horses that were pacers in their younger years are very difficult to get to trot normally. Most race tracks use chains, hobbles, ropes etc. to train them how to pace. Naturally, this is extremely uncomfortable and painful for the horse so in your horse's mind if he trots normally it will be painful. My suggestion would be to excessively reward him when he does trot, pet him and praise him and offer treats (within reason). Help him learn that it's OK and it's not going to hurt him to trot. The physical damage done by the track has created a mental block. This can be overcome but the older he is and the longer he was on a track will make it more difficult. Good luck to you both.