Help!! ~from horsey people~?

ok well i bought a black and white paint gelding, reining horse, monday an he's the best! ive been riding him for about a year an i got the money an bought him the only problem im having w/ him is b4 i bought him beginner students were riding him an let him get away with alot of stuff like when he'd slide they'd close their legs an make him side p or walk out of his slide an when they'd ask him 2 spin tehy'd make him walk circles inside of actually spinning. an he's been very jumpy lately an he always thinks that i'm gona be making him change his lead. an when i groom him off he always side pes wheni bring the saddle 2 him an when i brush him an hoof pick his feet he tries 2 bite my backside. an i kno that he didnt use 2 do this bcz when my trainer first got him he waz a well mannered reining horse an he'd nvr try 2 bite a person of sidestep out of his slides an spins an he'd nvr try 2 reach around an bite a persons foot. is there anyway i can get him back 2 his oldself??