Getting a divorce

I'm heartbroken. I thought we were making it work but last night I found out he blocked me on facebook so that he could chat with a girl he once cheated on me with (and I suspect he still is). He originally told me he deactivated his account, so he's been lying about that for around a month. He's also constantly drunk and locks himself in the bathroom all night. I should have been more awake. He had me believing so many lies. I'm also terrified of being alone after all these years with him. I really do love him but it's clear that this can't be fixed. This is going to be so hard and I have no idea where to start.