How can I win the guy I love back?

I would say just be honest, and give him reasons why you need him

Look i'm going to be brutally honest here

if he left you for another person then hes probably not going to comeback

but if you wish to pursue him still then talk to him to figure out them mistakes so you can change then maybe he will get back together with you. honestly though if you have to change who you are for anybody then its not worth it ... there is a guy out there who loves you for you and he would not desire you to change you just got to find him.