WTF!

been working on physics the entire morning and each question plays around with my brain like its playdough. its driving me insane!!!! so WHEN do we need to factor in gravity in a particle that is travelling HORIZONTALLY and when do we not?!?! w%#%^$%&%& there's a thousand and one formulas and a multitude of methods to each question... i honestly feel like just giving up. and it doesnt help that the book ASSUMEs you kno the reason behind each step. plus all other educational material i have are in chinese. GOD DAMN IT!