Need help regaining my confidence with horses?

about a month ago a girl brought her horse to the barn i work at and hes a big quarter horse. anyway she told me i could ride him so i did. while in the stall with him about to put the bridle on so he wasnt tied used his body to put me against the wall and cornerme. his ears were pinned back and i knew he was being aggressice so i quickly gave him a sharp kick in the side thinking hed back off but instead he just whipped his head around and bit my lip (which is now scarred). i took his cribbing collar (which was metal) and gave him a smack with it but he then used his head and threw me against the wall i got a concussion he then kicked me in the leg. his all happened in a matter of about 45 seconds then finally someone grabbed him and brought him out of his stall. i know i probably did something wron the minute i let him pin me but the problem is i now how zero confidence wih horses i dont know. i really want to be a vet but now i dot wantto ge in stalls with horses in unfamiliar with. any advice would be appriciated. thanks!