I need desperate help...........?

First off I am a good person and not a whore so what I am about to say please dont hate on me because you dont know me. The problem is that I have been with my boyfriend for 4 and a half years now been dating since freshmen year in high school. I love him very much but there was a time where things were strainded and i made a terrible mistake of giving into to another guy(again not a whore) I wish i never did it it was a horrible terrible thing to do and is really unlike me. the worst part is now my period is late and i am really worried that i might be pregnant and theres no way of knowing if its the other guy or my boyfriends. I love my boyfriend and dont want to lose him i hate myself for what i did and i really dont want to hurt and lose him. what do i do. please please help me.