I NEED help with trail rides!!!!?

so at my barn the head trainer wants everyone safe and not to go out on trails unless approved by her. i say thats fine but the only way im goin on trails is if i can prove to her i can handle my horse. now i've been in shows and i'm a good rider in the arena but when it comes to riding in the fields my horse gets completely jumpy and goes faster. i round his neck and do dressage out there so he'll feel comfortable but the second i let my guard down....off he goes. to try and keep him under control i also make him go but as soon as he gets crazy i stop him and disengage his hindquaters. it works but then it wont. my trainer(cindy) said she will help me and all but i woluld like some other advice.... summer is coming up and i dont want to be confided at the barn. any ideas?