Trying to let go of the past

How do you let go?? You loved someone so completely that when then just up and leave without telling why, or how they changed their feelings so drastically, how do you survive that???

Where to start

Life is complcated. I used to think it was so easy. I would love them and they would love me, but it doesnt always work like that  :(  I recently had my heart broken, ripped out and thrown away, and I didnt even see it coming.