Am I really inlove or do I just think I am?

Okay, relationship of a year, all along the road I knew i love him, there was never a problem till now.. We have alot of problems and we have both hurt eachother. He doesnt do it anymore but exactly a week ago, i slept wiht someone else.. Dont know why and i dunno where my head was.. And i could never tell him this, he wont break up with me but it will break his heart into a million pieces and he said that if i ever cheat on him again, he will feel like he wants to die but he wont ever be able to leave me.. If i am inlove, why would i hurt him like that and not even have the decency to tell him? Is it possible that a person who is 'inlove' would do something like that? Yes, my life would be completly misarable without him, that i am sure of 100 percent.. Btw, i am 15 and he is turning 18 ina few days.. And dont judge me, i know it is stupid but i already have a ring on my engagement finger.. Please anyone help?