Help me clear this mess?

Hello, I am an 18 year old girl. Recently, when my boyfriend just checked my mail account, he found a few chats of mine with some other guys which revealed that i loved that third person. This broke his heart and he asked me as to why i did that. But i had never spoken to anyone else in an intimate way than my boyfriend. A few hours later, i realised that it was my brother and his friend, who knew my pword had used my account to play around some guy of their school and all the chats were sent by him. But my boyfreind is not ready to believe it. He sys he loves me immensely and so do i and this thing has left him heart broken. Though there is no intention of break up, the relation is strained. Please him me out of this so that we can get back to the same relation that existed between us