People that i love

OK well i am 17 years old and i did something that will i never forgive my self i was over my ex's house and we where just friends at the time and we where talking ans she looked into my eyes and i looked into her's now me ant my girlfriend kind of never see each other and i needed some one my ex kissed and i wanted her but i was not thinking how do i say that i am sorry so my girlfriend and her family and love and trust me again we are back together and engaged nit my ex my Girlfriend but deep inside i hate myself for what i did i want thing back the way they where before all this i need an answer please i love her