How do i get her back???

me n mygf of 4 yrs broke up recently.. cos i wasnt paying much of aatention to her..so she left me.. i agree its was fault..another guy proposed her n she accepted as she was feeling very onely.. he wasnt a good guy so they broke upaft 3 months..we broke up 4 months ago...during tis time.. i've neva let her go..i've always let her know how iloved her n how sorry am i for ignoring her..now tht they have broken up.. i'm trying my best to get her back..but she doesnt wan.. i sent her valentine gift..n romantic lyrics..jus to let her know how much i loved n stil love her..i cant forget her n i cant go any second without her..vbut she's firm wit her decision n doesntwant me back.. we're frens now but i jus cant treat her like one.. i wan her back n i dono how to convince her tht she really mean alot to me..pls help i really wan her back..she's my heart n soul..she's my happines..pls help..i jus wana know how do i convince her tht i wil neva let her down at any moment