Why did I change so suddenly?

Sweetie, its all about growing up and maturing which is what's happening to you. It's just life and this guy was obviously not the one for you so stay strong and do not give him a second chance ok. Look at it this way, he showed his true colors and at least you found out now and not two or three years later after perhaps you were married and pregnant. Its ok to just care about yourself for a while, your young, get out and have fun before you consider getting married.